I'd start by saying Pelinor's guide on MCM creation is great. It's helped me come the long way from nowhere to where I can now fairly efficiently write up some menus.
You can find that guide RIGHT HERE
The very first step to creating an MCM is to create a quest to hang the menu on. I'd offer one important edit to his text. He writes:
Step 1: Create a quest.
a) Create a new Quest at Actor Data → Quest → Right-Click → New.
b) Make it Start Game Enabled with a low Script Processing Delay. (Values from 0.01 to 0.1 are fine.)
here I would insert:
b and 1/2) Check the box that says "Allow Repeated Stages" because if/when you get around to using quest stages to allow for bigger menus YOU WILL ABSOLUTELY WANT THIS AND IT'S A GOOD HABIT TO START
then back to Pelinor's guide:
c) Click OK to create the Quest.
He does tell you to do this in the section titled "Reducing Script Size". And the first mod I added a menu for worked just fine because I read that section as I worked.
The second mod I wrote a menu for would display the menu I'd created one time then nothing. If I switched to a sub menu or the menu for another mod then came back it would be blank. The solution required me to pound my head against the code for a couple days before realizing what the problem was. I hadn't checked that box. The menu stage would run once then since repeated stages were not allowed I couldn't display the menu again, change a value again, view a different mouse over text, etc.
Checking that box from the start, for this type of quest, is harmless and could potentially save you MUCH of the headache I've experienced this week.