The reality is that while most everyone wants a mentor, very few people have the confidence to be a mentor. Therefore, it is most unlikely that a homeschool mom is going to approach you with an offer to mentor you. Instead, you have to take the initiative to seek out that mentoring relationship.
The Direct Approach
You could simply ask outright, "Would you commit to being my mentor?" But many moms will be intimidated by the label of "mentor" even though they are capable of the role. This approach will work best with a very experienced mom who is very confident in her abilities.
The Indirect Approach
If you suspect your potential mentor would modestly decline to mentor you, you can try the indirect method. Ask her to meet with you somewhere for a cupcake or for coffee just to talk about homeschooling. You can say that you have some questions and want to get her advice. But don't use the M-word (mentor). Keep it informal and friendly.
If your first meeting goes well, follow up in a week or so with another invite. Then you can discuss any new developments or issues and tell her how her initial advice is working.
Over time, she may catch on that you would like a regular meeting time. If you keep it informal and avoid calling your mentor a mentor, you very well may get a lot out of the relationship.