I'm pretty sure e posure therapy is the best way, so probably worth googling that.
The good news is I think you can start by looking at pictures of spiders, and I would suggest small spiders like a common house spider, if you are severely arachnophobic this may cause some distress, so you will need to psych yourself up (its just a picture, it can't harm me, I'll only look for 1 second). Think good thoughts after looking, I'm fine, it didn't harm me, I'm proud I managed to look for 1 second etc. Then build up your time as you get more comfortable, I think I can do two seconds, again congratulate and celebrate if you achieve this, your fine. Keep going from there