Start with just getting comfortable with pressing s finger on your ass. You should feel it relax and open slightly. As above lube is your friend. But being comfortable and in control of your ass is better. Spit is a starter, I've even used butter in the past to get a finger in comfortably. Once you're ready put more and more in. This make take some time. If it doesn't feel good then don't do it.
Tl:dr. Press a finger against your hole, control, relax, lube, enjoy. Else just dont