nothing. let the baby grow and figure out herself. put baby on floor for maximum body movement and support. a baby can crawl upto 8-9 months of age. do no compare and do not push the child. the child will meet milestones when the body is ready.

it differs from baby to baby. some of them crawl and some of them directly sit. do not worry. my baby dint crawl too. she directly sat and she started standing with the support.

Check out milestones articles o babychakra