CS grad student here. Some mistakes I made were
Not documenting code.
Naming figures/datafiles poorly, so that you have no idea what they are in two weeks. It’s best to have an automated way of labeling files if you’ll be creating a lot of them.
Storing data in an inefficient way (very bad if you’re generating large amounts of data).
Not using version control.
Diving right into implementing an algorithm without first thinking about whether that’s the best way to solve the problem.
Being intimidated by tasks that looked difficult (they were rarely as hard as I thought they would be).
I think this is precisely because you aren’t charging for advice. If you charged a $200/hour “consulting fee,” people would see you as professionals, and value your advice highly. But now that you’re giving advice for free, you’re just some random Internet guy telling kids how to run their lives.
Well, you market it to the parents. People pay for college admissions consulting all the time.