I am slowly building a computer and buying the parts as I can afford them. When parts arrive, should I assemble them as they come or wait for all the parts.
You should be buying all the parts at the same time. You don't want to put it together and have it not work then find out you waited to long and missed the 30 day return window.
test them in case they need to be returned. But you will need certain minimum parts to be able to test. PSU/MB/CPU/Ram at a minimum. Video card as well if no integrated graphics.
You should save your money and just get the parts all together later really.
I recommend buying all at the same time if possible, 2 main reasons:
However, if you've already purchased some things, in order to test the core components for functionality at minimum you need motherboard, cpu, ram, psu. Use the motherboard box as a bench to set the board on, plug stuff in and make sure it works. If your motherboard doesn't have an on-board power button, short the power pins with a screwdriver to turn it on. Plenty of resources online showing how to do this if you're unsure.