My DD just passed the 11+. She needed a super selective level score because we are out of area and we did use a tutor. I do think its possible to tutor yourself, with a couple of things to bear in mind.
First, check carefully what your daughter will cover in school, especially for maths. There are definitely areas that come up in 11+ that isn't covered in school until year 6, so you will need to teach those areas, not just do practice papers. Eg DD hasn't yet done algebra at school and hadn't covered areas/perimeters or probability when she sat the exam.
Second, I think how well it works depends on how disciplined you are. DD did an hour and a half's class every Saturday plus the work that came out of that and then started doing 2 practice papers a week from Easter of year 5. I think without the regime of a regular class, life would have got in the way and we wouldn't have done anywhere near as much work with her. If you are the organised type, though, this shouldn't matter. Also, start now. Little and often from now is far better than panicking you are behind at Easter.
Third, think about teaching exam technique, not just topics. This can be really key. DD had one practice test with tutor where she blew the timing and got a really low score on the paper. This was a good thing - she learned from that in a practice and didn't have it happen in the real thing.
Good luck, whatever you decide. It was a really tough year, DD had to work very hard and worrying about whether we had done the right thing in putting her on the rollercoaster was very stressful.