You don't know how many times I see people lining the mounting block up to the horse all tidy like, then, just as they are about to mount, the horse moves, so they start all over; lining up the block to the horse instead of the horse to the block.
Or, they get on the mounting block, edge the horse up and when the horse goes too far, they hop back off the block, walk the horse around and start again.
Stay on the block, bring the horse to you!
When my trainer first showed me, I couldn't believe it was possible to do all that repositioning with ONE rein, and never change. But, it works.
To teach Mac to siddle over to the block , because you may get him all lined up and he shifts his hiney out, I used a dressage whip and reached over him to just tap tap annoyingly on the outer side of the cantle (as far as I could reach) until he moved in any way toward me. Release. Again, and on and on. I used the command, "over!" and now , all I have to do is stand there and kind ove wave my hand in a circular motion, and say "over!" , and Mac siddles over. He is a smart horse!