May just take some experimenting. It helps to be aroused before you start, so you'll need to get turned on. The prostate usually swells when you are aroused making it easier. Are you penetrating within your anus or from outside? It can be done both ways but inside is easier. Would probably really be helpful if you had a partner to do this for you. Also, you should pee before hand because stimulating the prostate may make you feel like you need to urinate. This article has a lot of suggestions for how as you scroll down the page. https://www.healthline.com/health/healthy-sex/prostate-orgasm#techniques. If you have arthritis, perhaps use a vibrator or sex toy.