It will come with practice, I promise!
You're so lucky to have the opportunity to practice on a real person before you go into the clinical to try it! We have these dummy arms that we were allowed to practice on for a couple of days, and that's nothing like sticking a real person. In my case, I practiced and practiced on the dummy arm, then my dad (who is a paramedic) brought home a couple of caths to let me try on him. I went straight through the "good" vein in each of his hands (was unsuccessful). I was so upset! My next chance to try it came in the clinical setting on an African American man in the ER. With my instructor hovering over me (and getting in the way, I might add) I got the IV on the first try. I was so excited - we didn't have many opportunities in this particular clinical to work on our skills, so I knew this would probably be my only chance at attempting an IV in this clinical.
As far as the tourniquet goes, it just needs to be crossed tight around the pt's arm, with the outermost piece of tourniquet tucked under the other part, with both end pieces sticking up away from your puncture site. (That probably sounded confusing, but it's the best way I can explain without showing you...hope it helps!)