Wiring LED strips in parallel & ground Schematic: 



I am making some light fixtures controlled by an Arduino UNO. Each fixture will host two approx. 1 meter long analog RGB LED strips with a current draw at full power of ~1.2 A per meter. I have a 12 V power supply that I will wire directly to the strips, with the RGB channels wired to the Ardiino PWM pins via NPN MOSFET (note in the schematic I've only shown one color channel for simplicity, but there would of course be three transistors hooked up to three PWM pins). The Arduino itself will be powered by USB.

My main difficulty in wiring the system is that for so much current I need fairly thick wire. The pads on the LED strips are delicate and I have several times damaged them by mechanical stress from the wire while soldering. The thick wire also crowds the narrow space between the pads creating a risk of a short. Therefore my first question is (bear with me, I'm pretty new to this):

1) How should I wire the LED strips?

My thought is to wire up the strips in parallel with thinner, more flexible wires. This brings up two sub-questions:

1a) How should I join the thinner wires to the thick wire coming from the power supply?

1b) At the junction between the wires leaving the lights and running to the transistor, does the wire need to become thick again? (I think the answer is yes – the current should be the same as coming out of the power supply.)

I also am unsure about the ground.

2) Does the wire coming from transistor to ground need to be joined to the ground pin on the Arduino or can it run directly to the ground terminal of the power supply?

Sub-question:
2a) Does the wire coming from transistor to ground need to be thick?

I don't really understand how transistors work (basically my understanding is that the Arduino uses a small amount of current over the PWM pins to somehow control how much current is allowed to flow through the rest of the circuit.

I've drawn a schematic showing how I think maybe this system needs to be wired up. Do I need to make changes?
I think you are overestimating the size of wire required to attach to the LED strips, I have connected a lot of single color LED strip (in excess of 1mtr) with the wire that was already attach and it was reasonably light gauge.
Look at this Example on Ebay and you can see what i mean.

I am note an Electrical Engineer so i don't know the method to calculate the minimum wire gauge, but if in doubt contact the manufacturers and ask them or search on the web.

As for the Transistor mounting/wiring you would mount that on a small circuit board with terminal block fitted for ease. Any power/connecting wire should not be less that used to the LED strip.

Also you will need a current limiting resistor attached to the Base pin on your transistors. You will have to work out the value of that based on the spec sheet.

There is a bit of learning required first, once you have researched the above you will have you exact answers.