Uncommon Second Job

I recently got a second part time job that requires me to fill out a W4. Annually I'll be making less in this part time job than my full time job. The job does not have a required number of hours to work for a week, month, or year and the pay fluctuates based on the shifts you pick up. How do I fill out my W4 for this job and update my W4 for my full time job?