School me on using a multimeter.

I thought they would all be the same, but I saw in b&q they had different types with different voltage ratings or something I think. 

User Guide Link:

When the guide refers to "receptacle", that's the American expression for mains plug socket on the wall.

Beware, the image in the manual showing the sockets for the leads is wrong when you compare it to an image of the unit.  On the photo of the actual unit the COM (common) black lead plugs into the middle of the 3 sockets and for use on low powered stuff like guitars you plug the red lead into the VΩmA at the right.  The user manual wrongly shows the VΩmA in the middle and the COM at the right.

Assuming your unit matches the layout shown below, then that is where you plug in your leads for this kind of work.  You would only swap the red lead to the 10A socket for dealing with much higher power like mains.

To answer your questions.

The miniscule voltages and currents in circuitry of a passive electric guitar are so small that you aren't going to burn anything up or electrocute yourself if you wire components wrongly.  It will either work as intended, work other than intended, or not work at all.  If, however, there is a fault with your AMP it could potentially zap you or burn something out.  All that's happening is the magnetic field around the pickup being disturbed by vibrating strings produces small electrical signals that travel through the cable to the amp and are amplified / magnified.

Your most useful dial setting is the "Continuity" one between the 12v BATT setting and the yellow 200Ω (Ohm) one.  The symbol with the small circle and 3 curved lines indicates sound.  If you touch one probe to one end of something metal, when you touch the other probe to the metal object it will beep to tell you that the object is electrically conductive between where it is being probed.  You can use this to test a wire inside an instrument cable by probing each end.  You can use it to test for a continuous ground connection in foil screening in the cavities or between the grounded side of the circuitry and the bridge string earth, or a continuous connection of the wire between a switch lug and a pot lug while the guitar is wired up.  There are multiple uses.

It would be useful if you could find some leads with the same plug-in ends but with clips on the other end rather than pointed probes.  You could buy leads with crocodile clips at each end like THIS and just clip one end to the pointed metal probes, or you can even just improvise by finding crocodile clips like the ones below and just push the probes into the part intended for a wire:

For the DC Resistance readings, choose the value that it's likely to be closest to.  To check the DC Resistance of a pickup, set it to 20K and probe the pickup leads.  It's unlikely you will have a pickup over 20KΩ DC Resistance and it's unlikely you will have one right down as low as 2KΩ, so for most pickups you will get a true reading with it set at 20KΩ.  For pots of unknown resistance, the 200KΩ setting should give you the accurate result.

Video showing how to test pots outside the guitar.

Measuring guitar pots while inside the guitar.

How to Measure Guitar Pots Taper & Resistance.

Other uses for the multimeter in electric guitars.

I neglected to answer one of the other things you asked in your opening post.

skunkwerx said:

...... I know of the words 'continuity and ground' but wouldnt have a clue what it means or how grounding works.

When thinking about your house mains wiring, you have a Live, Neutral AND an Earth wire in the cable that passes from one plug socket to the next and in the cables attached to the plugs you push into the wall.  In that kind of higher power electrical circuit, if you touch the live wire anywhere the electricity will use your body to find its way to earth and electrocute you.  Electricity always tries to find its way to Earth.  In mains wiring the Earth cable is present in case there is a fault somewhere and the Earth cable can then carry the fault current away to Earth.  The actual electrical circuit in a house only really needs two wires to work (although not safely), the Live and Neutral.  If you think about a table lamp, the cable from the plug to the lamp only contains those two wires and it works.  If you had several ceiling lights that could be turned on and off from one switch, you would have a length of cable going into one light, then another coming out and going to the next light, and so on in a big continuous loop.  The switch would just break that loop to stop the electricity going through the loop.

That's high powered electricity.  If you look at how a normal conventional car is wired, the battery has a positive and negative terminal.  The negative terminal is normally connected to the metal body of the car.  Where possible, instead of having two wires going to each and every bulb in the car, only a positive wire will go to one terminal of the bulb holder.  The bulb fitting will rely on contact with the metal of the car body to complete the circuit because the body is connected to the negative battery terminal, which is why miscellaneous electrical faults in older cars with rusty bits can be hard to pin-point.

In an electric guitar your pickups more or less act as a battery when their magnetic field is being interrupted and it creates electrical signal in the wires.  A pickup only needs two wires to work and that's what you get with a Strat or Tele pickup.  Where you have more than 2 wires on a humbucker, they will be the start wire for one coil and the end wire of that coil, and the start and end wires of the other coil, plus a shielded "ground" wire, and you can connect these in different ways for coil splitting, phase reversal, etc.

If you start with the Mono jack socket and corresponding plug of the instrument cable, you have the Tip of the plug that's treated as the Live or Hot or Signal connection, and the Sleeve that's treated as the Ground.  That's a TS (Tip/Sleeve) plug.  A stereo plug (TRS) has two separate sleeves and one is called a Ring (Tip/Ring/Sleeve).  Your guitar circuit isn't a complete circle.  The jack is more or less a switch that breaks the circle.  When the cable is plugged in and connected to an amp or other device, the cable just continues the wiring outside the guitar and at some point down the chain the circuit is completed.

Rather than having two wires skipping from pickup to one component, then two wires skipping to the next component, and so on, an electric guitar is in some ways like a car with a negative earth.  The Hot / Live wire will do the skipping between components and end up at the tip connector of the jack, but the other wire from the pickup will connect to one component.  That component then relies on a common ground connection between all the other components and eventually ends at the Sleeve connector of the socket.  A break in that continuity will result in it not working or not doing what it should.

In a Les Paul that uses wire with an external wire braid screen, you will see this braid soldered to the back of each of the pot casings as that cable snakes from one to the next.  That is being used as the ground wire to connect each pot to a common ground that eventually leads to the Sleeve connector of the jack.  The separate wire core inside that braided cable only goes to the lug of one pot and to one terminal of the toggle switch.  The braid at the switch end will be connected to the ground connector of the switch.  When you see a leg / lug of a pot bent back and soldered to the pot casing, that will then be grounded so that the pot works as intended.

In a Strat you will sometimes see a wire (most often a bare one) connecting all three pots to form a common ground, however sometimes this isn't present.  Instead they are all relying on contact with the metal shielding on the underside of the scratchplate to create the continuous ground, and this can be unreliable  That's what happens with the metal control plate of a Telecaster and there is no need to connect the pot casings together with a wire and the connection is usually a lot more reliable than metal foil.

There's various ways to create a common ground connection in a guitar.  Some use one central point like a screw into the shielding paint for all the ground connections from the components to connect to, while others wire the pot casings together with a wire that skips from one to the next.

The above is very simplified but hopefully it answers your question about grounding.  With your multimeter in Continuity mode you should be able to touch the body of your jack socket or the metal jack plate with one probe and hear a continuity confirmation beep as you touch all other components that are connected via a common ground, like the pot casings, the nut on the switch, the bridge, the nuts on your pots, etc.