My son says the following:

First of all and most importantly, it depends on your school. But in general, if you are granted work study hours, you have to see which places on campus are advertising that they are seeking work-study students. And see if they are interested in you. You yourself would need to take into account your course times.

Students can appeal for more money if they are not happy with the financial aid package they received.

Labs don't tend to advertise for work-study students. If you are interested in a lab position, then you will need to contact the PI (primary investigator) of the lab projects they have going on. They might be willing to take on a lab worker, depending on their needs and whether they think you can do the job adequately. (They really aren't there to train you much, in other words.)

My son goes to a very large, private university. Things will be less complicated, I would think, if you are going to a very small school.

Hope that helps some! Let us know how things go!