You can pick writing jobs directly from a live job board or have to bid on a job and hope that the job is awarded to you by the relevant client, depending on how the site works. Some sites may require you to take an English grammar and writing test and some may require you to submit a sample or samples of your work while others may require you to take a grammar test to evaluate your skills. If you pass any relevant test, the relevant site will let you take on writing jobs.
The best money-making move I’ve made so far at uni is to sign up to a few ‘extras’ agencies. You do literally nothing (just a bit of standing around on camera) and you get paid extremely well, often over £100 a day. You also get to creep on celebrities, which is always fun. If I had to choose between that and working in a busy pub, I know which one I’d go for.