As noted above, learning the AGC (Apollo Guidance Computer) is the real trick. It is not user friendly, displays what seems to be very cryptic information, is not consistent in it's input and output, and, almost to a man, the astronauts didn't like it. The good news is that if you ask on this forum, there are folks who know this thing pretty darn well, and will tell you what to do. It's a good thing too, because, so far, I haven't found anything like an AGC for dummies, like an astronaut probably would have wanted. What they had, it seems, is a lot of training, augmented in flight with operation checklists that had step by step instructions on what to load into the computer for each program.
To get a good head start, try:
https://history.nasa.gov/afj/ where NASA maintains a lot of documentation mission by mission. When MCC produces a PAD for you, that is all info related to what you would put into the computer for a specific action, and the flight plans have forms that you would write this information on. Some of those flight plans are on that site, so you can print those forms.
In real life, he PAD would tell you what you would put into the computer for maneuvers you need to make sometimes, but also for contingencies, where something went wrong, so some of these you wouldn't use. I don't know for sure if every contingency maneuver is included, but you can tell: For instance, if you are going around the moon and you get a TEI PAD when you wouldn't actually expect to need to blast back to Earth, it is for a return if something goes wrong on that rev. Basically, a mission abort.
The format tells you what the PAD is for. For instance, the first PAD you get in orbit on Apollo 8 is a T+90 abort maneuver pad. It is SPS G&N, so the service module engine will do the burn using the guidance and navigation system. It is fallowed by what may seem to be an incomprehensible list of numbers and letters. Using a pre-printed form will help you here, because you'll generally just need to write in the numbers.
Now, to make some of this a little understandable. The AGC runs programs (PXX), and you input verbs (VXX) which usually mean do an action, nouns (NXX) the object associated with an action, and registers (R1-R3) which are the 3 lines at the bottom of the display. Registers display information, allow information to be input, and are in the format +/- XXXXX, with no shown decimal. On registers, the information often gives you a result in tenths, hundredths or even thousandths, and it's up to the user to know this, because there is no decimal on the display, so the PAD forms have a decimal written in, just to be helpful.
So, back to the PAD. This T+90 is shown as a P30 PAD. That means it's information you put in once you start a program 30 (maneuver). There is a very important verb you need to know at this point, and that is verb 37. This tells the AGC "run program NXX," so if you are in P00 (idle) or "pooh" if you hear it on the ground loop recordings from the missions, you would enter V37E (run) (the E is enter- don't hit enter unless the listing says so) 30E (program 30 maneuver, and the noun is implied i the program). Fun, yeah? Ok, then you get into the numbers. These are then entered into the program by following down the PAD form, and entering the NXX on the far right of each section. The checklist looks like this:
V37E 30E
F 06 33 GETI (this is the part of the program you are on, routine 06 N33) [this checklist assumes a P52 has given the trim values]
Load GETI (ground elapsed time of ingnition)
F 06 81 DVXYZ(LV)
Load desired DV's
... etc ...
That is generally what you get from the MCC pads, and I hope this demystifies the AGC just a little. I still only understand it so so, and I think that is about par. Reading NASA memoirs, I see the phrase, "drinking from a fire hose," over and over, so I don't feel so bad for feeling the same.
