If you just cut off the testicles, you will cause such severe haemorraging that you could even kill the horse. Vets go through a complicated procedure to remove them and sometimes just crush the spermatic cord without removing the testicles.
Whichever method is used, it is absolutely not a job for anybody except a competent, qualified vet. Do not risk hurting your horse and yourself by trying to do it without the proper knowledge and equipment.
Do not try to do this on your own. I have seen a vet in the process of gelding a horse. it is difficult even for trained vets. if you try to by yourself, even if you are a skilled horseperson, unless you are a vet it is not smart at all to geld it yourself. Call a vet. you and/or your horse would go through horrible pain or death. consult avet