What’s the things i need to know before starting DIY?

Hey guys another post again. So i recently looked into DIY as it’s my only option, but i’ve read the google doc of diy hrt and the website for it, but i’m not really understanding it much. All the science words and stuff confuse me so much. I’m aware of what i need to purchase, but what are some things i should know and educate myself on before jumping into it? Like there’s different needle sizes, where to inject it, etc. Just looking for anything that i should know before jumping into this.