First of all, you want to make sure that every resource you've used from the RTP is in your game folder. This can either be done using the 'Import' feature in the materials database, or you can find the RTP folder on your computer and copy/paste everything in your game folder.
Then, after it all there, go to Game -> Select RTP and change it from 'standard' to 'none'.
Next, go to File -> Compress Game Data and choose the file path. Once it's finished compressing, find it in your folder.
The easiest way to do this would be to use a .zip or a .rar archive. You'll need to include the file(s) in your RMXP folder that begin with 'RGSS' and are a .dll format. (Most people have only 1, I have 3 and so do some others). Zip those all together, and then there you go.
Make sense?