I'm not really sure this falls under software recommendations as much as software instruction, but since it seems fairly easy, I'll tell you what I found.
First of all, I simply googled "jgrasp to jar".
There are youtube links that claim to tell you how to create an executable jar, which may be what you want directly. Personally, I HATE video instructions. But if they work for you, they do exist.
The first actual link was a walkthrough showing how to use JGrasp to create a jar file.
I've never used JGrasp, so I'm not sure how helpful this will be, but it looked fairly thorough.
Again, as you are new, you may not be aware, but there is a good chance this question may get flagged as off topic, since you are asking how to use a program, not what program to use. There are other Stack Exchange sites that are better for support.