This Extreme Reactors mod guide will get you started with a basic reactor setup.
PLEASE NOTE: You can also use the big reactor simulator located here: https://br.sidoh.org
Items needed for my Reactor:
Reactor casing x 76
Reactor glass x 96
Control rod x 5
Fuel rod x 20
Reactor access port x 1
Redstone flux power tap x 1
Reactor controller x 1
Yellorium (this is fuel, you will need a few to get started)
Step 1: Make a square or rectangular base for the reactor. In my guide, I have used a 6 x 6 base.
Step 2: Place the Reactor controller, Power tap and the Access port on any of the sides.
Step 3: Build up the outer frame using reactor casing
Step 4: Place fuel rods in a pattern like below.
Step 5: Stack the fuel rods until you reach just below the reactor top. Add control rods on top of the fuel rods
Step 6: Use reactor glass or casing to make the walls of the reactor complete.
Step 7: Through the access port, input yellorium into the reactor. The fuel rods will turn yellow.
Step 8: Fill the reactor with a coolant (diamond blocks are the best I believe) and turn the reactor on using the controller.