Each of the fuel cell blocks that you placed inside your reactor will take some Yellorium ingots, but I don't know how many ingots per fuel cell. Basically, you just put two ports on the side of your reactor, change one of the ports to the blue color, leave the other one green - now you have an input and an output for your ingots. Just keep dumping Yellorium ingots into the input port until it is full with 64 ingots. At this point all your fuel cells will be full with Yellorium, and you have a reserve of 64 ingots which the reactor will slowly use up over time. As the fuel burns up in the reactor, it will convert to Cyanite ingots and be placed in the blue output slot. If you don't collect those, the reactor will shut down when 64 Cyanite are in the slot so make sure you automate the collection. Of course, you should also automate the input slot as well, then your reactor can operate unattended, pretty much.