Hmmm... interesting question. Because it is very easy for me to regress, I always assumed that most AB's simply regress naturally, that there is no rhyme or reason or general rule of thumb to it, you either feel like regressing and being a baby or little or you don't, it's not something that can be forced. If you are naturally inclined towards being little and being a baby even in the slightest, then regressing is or can become second nature, so, I'm not sure if there is any surefire advice or first steps I can give. There were no first steps for me, regressing has always been a natural inclination and when I feel like babying out and regressing I just do it, it doesn't take much for me to slide into baby-mode, sometimes just throwing on a diaper is enough.
That being said, If regressing is something you naturally and genuinely want to do, just throw on a diaper and take it from there, don't rush things, give it time and you'll regress, think of childish things. If that doesn't work, I guess you could try surrounding yourself with childish/babyish stuff, that helps me to remain in baby-mode when I regress and who knows, the things you enjoyed as a kid could serve as a trigger for you, a regression aid so to speak.
So, ask yourself, what are some things that naturally make me feel little? What are some activities that could help me to regress? When you figure it out, try having those childish/babyish things around you and try doing some of those activities, that might help you to regress, drop into and maintain little headspace.
At the end of the day, regression is different for everyone, it comes more naturally to some people than others and everyone regresses a little differently. With practice you might find regressing becomes easier over time and who knows, maybe you're a regression natural like me, you just never got around to trying it until now.
I hope my advice can help and hopefully, some other AB's on the site can give you more tips and advice. Good luck to you