You need to decide the following things:
Name: Exactly what it says on the tin. Name of the plane.
Apperance: What it looks like. Is it empty? Full of rocks? Colorful? Dull?
Gravity: How much anf in what direction, if any. Your options are
-Normal Gravity. Exactly like our universe.
-Heavy Gravity. Like out universe, but more of it.
-Light Gravity. Like out universe, but less of it.
-No Gravity. Nothing has weight, stuff just floats.
-Objective Directional Gravity. Gravity doesn't point towards massive objects, it points towards or away from some specific direction, location or object. Istamoshor is an example of this, gravity points away from the center of the plane
-Subjective Directional Gravity. People decide which direction they fall. Every individual chooses a "down" and then they fall that way.
Time: How long is a second on your plane compared to a second on prime? Longer? Shorter? Random? No time passes per seconds on Prime?
Shape and Size: How big is it and in what direction.
Infinite: Like our universe.
Finite: Has walls at the edge of the universe
Self-Contained Shape: You go far enough in one direction, you wind up in another.
Random: There aren't directions, going a set distance in a direction will result in you reaching a random location.
Other: Just make up how directions work. Diminsonallity is very complex.
Morphic Traits: How do the physics work?
-Divinely Morphic. Things work how you say they do. You wave a hand and the plane is now made out of cotton candy.
-Alterable Morphic. Things are normal. Things move when you push them and stay still if nobody is pushing them. Objects tend not to burst into flame without a good reason.
-Highly Morphic. Like Divinely Morphic, but for everybody. Anyone can walk into you plane and declare it is now made of icecream.
-Magically Morphic. Like Divinely Morphic, but only to people with magic. You have to cast a spell to make the trees start dancing.
-Static. Nothing changes, even if it normally would. If you do manage to change something, it changes back when you stop watching it.
-Sentient. The plane has a mind of it's own and is Divinely morphic to itself.
Alignment: Planes can have Alignments just like people. If evil people walk into good territory, they feel ill. If they are really really evil and the plane is really really good, they burst into flames.
Magic: On some planes, certain kinds of magic work differently. For example, on the Elemental Plane of Water, it is very difficult to get fire magic to work.
Layers: It's sort of like having a second floor to your plane, except instead of stairs you have portals.