Drink water beforehand. Take a baby aspirin if you can take aspirin. It thins the blood and makes the process faster. Alcohol consumption makes it take longer. Iron levels are important, as too low will get you deferred for the day.

Typically the first few donations are 'worth it' compared to afterwards. I Think most places offer some sort of bonus for the first few donations. Twice weekly is max for my area, though I'm sure it's similar everywhere, plasma takes time to come back up.

Big ass holes in your arms. The needles aren't small. If you're squeamish about needles, consider passing. I quit donating plasma over a decade ago and still have scars from repeated needle sticks. If you are not comfortable w someone seeing them and possibly assuming you use, consider skipping. Full blood work up for communicable diseases and drugs harder than weed where I reside, ymmv.