I am in the same boat, trying to learn something new without the input of a trained horse or good trainer to direct me and give me the proper feel, timing etc. To make matters worse there are a lot of professionals out there that can ruin a horse much faster than I can because they don't really know what they are doing. I am far more cautious to avoid making that serious mistake.
Trying to learn the spin, I started with ground work teaching a turn on the haunches. This is what works for me. Touch the nose to ask the horse to step out and over with his outside foot. Then bump him behind the elbow as his inside front leg (next to you) is lifting off the ground to ask him to step across in front of his other leg. If he wants to swing his hip toward you, tap it with your whip/stick to remind him to keep it planted. Just a step to start, followed by praise. Then walk him forward and start over.
There is a great video on another thread that explains the mounted way of getting the spin correct so it is a forward movement without the hind swinging out. Starting on the ground will help you and the horse understand the timing. Looking at the video, I believe now, it would be a good idea to keep the forward movement doing a few steps of walk and then a turn, walk, turn, etc.
I was just asking for a complete turn which my horse was doing on the ground and mounted but he had no momentum. It is often sluggish. I tried to get help but discovered the people "in the know" had actually sent their horses to another state and they didn't really know how to train. Seriously.
One person told me to just wear sharp spurs and jab the #^$% out of my horse until he did it. I asked what the timing was and the response was, "what timing?"