Use lots of water- or silicone-based lube.
If fingers are going to
be inserted in your anus, fingernails need to
be well trimmed, smooth, and clean.
Some people like to start with a clipped nail finger in the anus, moved very slowly inside. When there's pain, stop and wait a moment and breathe. This will allow the internal anal sphincter to relax. Once the discomfort stops, continue. If a finger is comfortable, you can proceed with other sex toys or a penis.