There is far more flexibility to sit whichever exams your daughter wants whenever she wants to do them than if she were at school. So you don't really need to be thinking, "She'll be in Y10 so she needs to be doing the first of two years of GCSE study ready to sit exams in two years' time" or anything like that.
The flip side of that is that all that choice can feel a bit overwhelming. So many decisions! When to do maths, which exam board to use, how to prepare for it?
You could start out by spending some time with home educators in your area to see what other people are doing. They will be aware of which local exam centres are available to private candidates, and at what cost - but do be warned that this often changes from one year to the next. There may well be in-person tutor-led study groups your daughter might enjoy. Usually those are done over just one year, to allow kids to really focus on the subject and sit the exam and then move on to other subjects. Juggling ten subjects across two years and sitting all those exams in the space of a few months, as schools do, is rarely the preferred method for people who have a choice!
One common approach is to sit just the number of (I)GCSE subjects needed to get onto their chosen college course, usually five or six to include English and maths. If your daughter has an idea what she wants to do, she could work towards that. Spread over two years, that is manageable for most kids. Some kids take longer.
However, the main thing is to consider what is best for your own child. Neither of mine sat exams at 16. One sat just a couple of GCSEs at a later age and then went to university. The other is 17 and is learning practical life skills; she might do Functional Skills exams later because I doubt GCSEs would be within her grasp. Without school to insist that teens do the same thing as everyone else at exactly the same time and in the same way, there are all sorts of paths they can take in life.
If your daughter was quite miserable at school, she might need some time away from formal learning to recover and feel ready to get stuck in. You could start off by just letting her do the things she enjoys and wait for her to get her spark back. Visit some museums, plant a garden, learn to skateboard, play the guitar - whatever makes her happy. Mental health comes first. Unlike at school, she doesn't have to "keep up" with anybody.