With my friends 2 year old paint, we wanted to introduce him to the tack early before we tought him too lunge. So I let him sniff all the equipment then softly set it on his back and moved it all around his abdomen.
Start with ground work first. The horse needs to trust you before you begin showing him a scary saddle. So join up or lunging will do. Once he respects you on the ground, then introduce him to a bare back pad. Let him sniff it at first, put in slowly on his back from the left side, then the right. Make sure whatever you do, you do from both sides! Then if he's okay with the bareback pad, lunge him in it. Then introduce a saddle and pad. Repeat the same steps with the bare back pad. If he's okay with the saddle, lunge him with it on so he can get a feel of the saddle and stirrups. Once he's okay with the saddle, gradually build up the weight you put on the stirrups until you feel comfortable and you think he's ready for you to hop on:)
If you have any questions about this feel free to ask!!
It honestly depends on the horse and his behaviour/personality. With a spooky horse you need to take all the steps real slowly. Start making them sniff it, then do everything like Willow K said.
If the horse is quite confident you can start lunging him with a roller and then switch to a saddle and pad (without stirrups first). Be always sure the horse is ok with what he has on before introducing something new.
Start by desensitizing them to a saddle pad until they're okay with it, then start by just putting your arms over their back and putting your weight on them without lifting up. That will get them used to a heavier weight on their bsck. Once they're good with that, put the saddle on with no stirrups and then take it off. Keep doing that until your horse is okay with it. Then put saddle (still no stirrups) on with girth and get them used to it. Once they're good with that start lunging them in the saddle with no stirrups. Then move onto lunging with saddle with stirrups and then once they are good on the lunge line like that you can start getting into harder stuff like riding.