Could you give us a better idea of what it is you're having difficulty with?
I don't have the books on me atm, but it's been my experience to build the rest of the ship first, then add up how much power I need, and choose an appropriate size of plant. Most of the time, "appropriate" means a Fusion plant that's slightly larger than the requirement, generally because I like round numbers. However, some designs won't have that much to spare; a huge power plant may add so much to your weight that your maneuver drive no longer does what you thought it should, so you may have to put in a bigger drive, which needs more power, which needs more power plant, which might need a bigger drive...... and so on. So if you already put in something bigger than you needed, you can usually tweak down instead of up, and it's usually easier that way.
OTOH, you should be checking, before you declare your design final, that your numbers are within proper tolerances. Adding power plant adds mass, which must be counteracted by drives; bigger drives have mass and require power, so you see it's cyclic, but there is a point at which you can have both being sufficient.
At any rate, several of the official designs had a "power shortfall", requiring the player to budget his energy. Most of the time, this meant turning off stuff you don't normally need turned on anyway. Frex, you really only need CG when you're near a planet, and you don't need to be powering your weapons at that time. OTOH, when you're in combat, you want the reverse. No sense in powering both all the time, just provide enough to power the one you need. If you happen to need both, then the power has to come from somewhere, usually from reduced maneuver drive output or lesser rate of fire.
If this wasn't helpful, please try posting a better description of what the confusion is, and we'll try again to help you out.