https://moguls.myfootballnow.com/gameplanningOffensive PlaysFor me personally, if an offensive play is averaging less than 3 yards per call, I will remove it and select a new play. For the most part, finding a good play is basically just guess and check. You won't know if a play works until you try it out. Sometimes, you'll have a play that works great one season, and the following season, it will be averaging 1 yard per call in the first 4 games. So always be checking your plays after each game to see what works and what doesn't. I don't know if you need to select 40 plays, but I've had some weird things happen where if I select under 40, the CPU will fill in the rest with plays that I had never selected before. Another word of advice, make sure you filters are set so that you can see all your plays. There was an incident earlier where a user had their filter set to only see plays that were used a minimum of 20 times. This caused them to only be able to select about 15 plays until they figured out what was going on.
Defensive PlaysI would suggest you select a few Goal Line plays, a few Quarter plays, and split the rest to Normal, Nickel, and Dime. Reason for this is that most teams will run most of their plays with 2-4 receivers, so this will cover most situations. I will generally replace any plays that are giving up 8 or more yards per call in the last 6 games. As with offense, defense is just a guess and check to see what works and what doesn't.
RulesRules allow you to have more control of the gameplan. If you want to always run on 3rd and short, you can set a rule to do that. If you want to blitz 2+ linebackers on 3rd and short or put your defense to protect against the pass on 2nd and long when you are up by 4 in the 4th quarter with 7 minutes or less left, you can set a rule for that. You can set all types of rules for all types of situations and this will allow you to dictate what your team does on the field. It can come quite in handy in the playoffs, when you set rules to combat your opponents gameplan.
ScoutingIt's important to always scout as this will give your offense and defense a boost when your opponent selects a play that you've scouted. It also gives your QB a chance to perform a hot read, which usually results in big yardage plays. I generally set the filter to the 6 most recent games and average per call. I will then select the top 10 plays that my defense has lowish familiarity on.
Watching Your GameEverything you do above culminates in the game, which I highly suggest everyone watch. Not only does it allow you to see how your team does, but it is definitely the best part of this website. It is quite fun to see your gameplan in action and also allows you to see what is working and what isn't.