First of lets check the requirements:
A normal Chest
A Oak_Sign (By default server owner can change this)
First start building the rocket as u wish. keep in mind the rocket may not extend higher then 15 blocks above the lauchpad and may also not be bigger then the launchpad.
Now make sure there is a chest in the rocket thats reachable to put fuel in.
Write down the coordinates of the middle block of the launchpad.
And last step go to the area where u wanna stand at launch (cockpit)
Place the oaksign against the wall and write the following:
[rocket]x/y/z(where XYZ are the coordinates u just wrote down)
if all goes well and u click the sign it should give you either a error if something is wrong or a final score!
if the score is on the sign u can also see the base fuel usage on the last line of the sign if u put fuel in your chest and click the sign again you can start launching!