A mentor is not necessarily an ongoing relationship, but a person who is doing what you want to do - but more successfully, or a few steps ahead of you - that you can ask for a piece of advice.
Gretchen McCulloch breaks down the best way to do this, and I love it because a) I reckon it's spot on, and b) I love when people break down social relationships into actionable steps, because honestly it is not always that intuitive! Some guidelines help!
1. Say: “I’ve done X and Y towards Concrete Goal, do you have any suggestions for what I should do next?”
2. Do the thing they suggest.
3. Report back to them on how it went (even if it went badly).
I think this is perfect. You need to, with step 1, show you've done the obvious things that would be on the front page of google. It's offensive to ask someone to take their time writing up something you could just as easily have googled yourself. Also, it shows you're actually going to put the work in, so it's worth their time to offer help. People generally are
happy to help others in their field, but they don't want to help the kind of people who are always talking about doing something, but clearly never really going to act on it, because it's annoying and makes you feel used.
Reporting back is gratifying, people like to know that advice they've given has been acted on. Again, it also shows you're serious and are worth taking the time to give advice to. So reporting back is not annoying them with a second email, it's rewarding them for the time they spent writing the first email.
That's the gist, but honestly if you're thinking of doing this you should read the whole post about it