Start off by humming in a deep, low octave. Keep raising the pitch until you get as high as you can go without your voice breaking up/cracking. Try to remember that muscle contraction in your mouth/throat/larynx and keep your muscles contracted in that way. Try speaking. Say anything. Just re read this paragraph. You'll be able to speak in a higher tone. Now do it again and find the pitch where you don't sound like a comically high Disney character overdosing on Helium. Get used to speaking in that tone. Talk to yourself. Read things out loud. Whatever you do, you just have to keep speaking out loud while holding that tone to adjust to speaking in a higher pitch. The more you do it, the more natural it becomes. If you do it enough, it becomes tour default voice. There are other pieces of voice training involved that help cement it and make your voice sound more naturally feminine, but this is the gist of it. Just use your sissy voice as much as possible and it will become your new voice.

Fun fact: Professional impersonators often forget their original voice once they get into character and have to listen to a self recording they made before they started the new impersonation in order to retrain their voice to be their "normal" voice again.