To start going to planets, first you need to find them!

Step 1: Make an Observatory
Step 2: Put cameras and lenses into it
Step 3: Research the "Find Planets" technology
Step 4: There should be a message saying something like "You found Planet ...!"

Now you can go there!

Step 5: Acquire a rocket
Step 6: Research "Space Survival" Tech
Step 7: Make a "Space Capsule" and put it in the rocket cargo slot
Step 8: When the rocket is finished launching there should be a menu
Step 9: Click on where you want to go, then click "GO!"
(The home planet is called "nauvis", so not there)
Step 10: You should now be on a different planet!

If you want to go back,
Step 1: DO NOT destroy the "Landed Rocket" or you will not be able to get back!
Step 2: Put 100 Rocket Fuel in the "Landed Rocket"
Step 3: There should be the same menu as when the rocket was launched
Step 4: Click "nauvis" then "GO!" to go back home

Suggestion: put this info on the mod page and include at least one image of what another planet looks like.

Yes, that (and more) should be written on the main page of the mod ;-)
So, if I understand it, you won't be able to go back to Nauvis before you're able to craft rocket fuel, isn't it ?
Are you able to carry items with you during the space travel ?
Awesome idea for this mod, seems to need some adds to be a real fun, mainly with material transport between planets. Maybe an upgraded cargo rocket through research, that needs to already have traveled, and that can load a lot of things ? Continue your hard work, it can become a great mod.

You can bring items with you so you could just bring 100 rocket fuel to go back