Make sure he has good groundwork training. I found helpeful a book called 101 ground training exercises for every horse and handler, by cherry hill, completly worth every penny.
Groundwork helps the horse to prepare for the future and it is even recomended for trained horses.
Make sure you have someone experienced guiding you, or teaching you
It really depends on the horse, cause they are all different to each other and react to new things differently. You need someone with experience to help and guide you because only experience can tell you what way to do it.
Anyway generally I start with getting the horse familiar with what you are going to use during the training. So the arena, the tack (bridle, pads, saddle and girth. boots come later), the lunge line and lunge whip and so on. I started mine with grooming and feeding, so he could trust me and know that everything new was fine cause he would get treats and scratches.
Then I started lunging him with headcollar and lunge line, and that took quite a bit cause he needed to understand what he had to do, without getting tired or stressed. Little progress at time.
Then we put bridle and roller on (he is really confident and brave, and didn't mind having stuff on), but with a spooky horse you might need to add one thing at time.
Then switched the roller to pad and saddle (with no stirrups) and as he was really relaxed we started to hop on (first with belly facing the saddle, and then straightening up).
As he didn't care at all the following day we put stirrups on the saddle and start walking him round, with someone holding him with the lunge line. After that he started going on his own and from that point he has gone better everyday.
Remember I am using as an example a brave and confident horse, really quite minded that made the job very easy.