The area to be contracted is the physical trigger point of the mooladhara chakra. This has a slightly different location in men and women.
For males: in the perineum, between the anus and the sexual organ.
For females: at the cervix, where the vagina and the uterus meet. Moola bandha is often done incorrectly by contraction of the anus. Try to find the exact location that we have described and apply the contraction at that point during moola bandha.
The best asanas for moola bandha are siddhasana for men and siddha yoni asana for women4. While sitting in either of these asanas, one heel applies firm pressure in the region of the mooladhara chakra, which improves the physical contraction.
If you cannot sit in one of the above asanas then you can sit in any other meditative asana where the knees rest on the ground. These include padmasana, swastikasana, vajrasana or ardha padmasana.