When the job becomes active, your screen will change and tell you the job your are debugging is active. At that point, do F10, Then you do DSPMODSRC and put in your break points. Now F3 twice and the program will be running.
REGISTER or login:
Note that you don't debug a "job". You debug a "program". It doesn't matter if a "job" is active or not; it only matters if a "program" that might or might not be called in some job is active or not.
So, first, you can't debug a program that hasn't been enabled for debug. Second, you must use STRSRVJOB to begin debugging a program in a different job; so you need to ensure that you have the proper authorities.
Those two requirements alone make it difficult to list the steps.
However, you should be able to clarify your question by going through the Reasons for using a job to debug another job topic in the Information Center. One of the listed scenarios should fit your requirement. Once you determine which one is appropriate and look at its steps, see if any details need to be clarified for you.