Follow her lead.
If she's deeply interested in something particular--say bugs--do what comes naturally as a parent. Expose her to info about bugs through the library, your local park system, a butterfly show at a conservatory, the zoo, etc. Play in the yard and look at bugs. Watch TV shows together about bugs. If she asks questions about bugs, do your best to find the answers. Draw bugs. Pretend to be bugs. Make bug crafts. Whatever she likes, do that.
All of that is fun and it's coming from the child. It's not "sit down and learn" time.
When she's interested in something new, go with that. Don't keep on about bugs (or whatever) when she's done with the topic. Move on when she does. (Usually this happens right after you spent a bundle on bug-related birthday presents, BTW. LOL!)
Other GT kids have more general interests, wanting to learn a little about everything. But the same rules apply: follow her lead and give her what she asks for. Quit when she's done.
In my experience FWIW, homeschooling isn't something that most parents of GT kids "start," per se. It's just part of what you do because its what GT kids require, just like they require food and water. Even GT kids in a bricks and mortar school are frequently afterschooled because their parents are meeting their needs.
I'd say that your telling her the letters is homeschooling, but we don't usually call it that at age 2 because it's just good parenting. Your child wants to learn, and you're teaching. It's child-led, so it's the most natural thing in the world for both of you. That's ideal!
