Quote Originally Posted by icosahedron View Post
Nexis, dunno if you were replying to me or GW here, but for the calculator I linked...

1. Decide roughly how quickly you want your moon(s) to orbit the planet (how long is your 'month'?) and how quickly you want your planet to orbit its sun (length of year).

2. Google for typical masses and distances in our solar system to give you some ballpark figures.

3. Trial and error some inputs of mass and distance to get a period that's about right. (distance can be used to tell you how big the moons will appear in the sky and whether eclipses will occur, if that's useful to you).

4. Start at a particular 'time zero' (perhaps a few months before your campaign starts) and place your planet and moons all at 12 o clock to the sun.

5. Use a spreadsheet to figure out how many revolutions and fractions of a revolution each body will have made by 'time X' and sketch the positions out on the back of an envelope.

6. Look at the relative positions of the sun and moons to figure out what phase each moon will be showing at that time.

Or...

Google for some calculator that might do the whole thing in one go.

A lot depends on how important this is to you and how much effort you're prepared to expend to achieve it. No pain, no gain, as they say.
Hate to admit it but I have never used a spreadsheet in my life. LOL. OK time to find out.