If I can get mom to use essential oils, how do I mix the oils?

If your dealing with a rep from Doterra or Young Living ( the 2 main ones ) they will help you with this, and have lots of good suggestions

Yes, always use a carrier oil, as most essential oils can be too harsh to use straight on the skin. Sweet almond oil is not too heavy and has little scent of its own; grapeseed oil is another good one. I use coconut oil often as well.

A few drops should be fine in a few tablespoons of carrier oil.

What did you want to use the essential oils for? And which oils? That will guide you toward what carrier base to use. Also make sure of any contraindications with any of her medications, and that you leave them out of her reach if she might use them inappropriately (ingest, put in areas or on wounds they should not be on, etc ).

baobab oil is good carrier oil. the other things is much depends of what you want to achieve in the end. But generally use 25 % of each selected ones.