Bettas like lots of plants in their tanks. It simulates their natural habitat

Also, provide little caves or rock formations since they like to have hiding spots. Terra cotta plant pots on their sides may nice little caves
Gravel isn't necessary. I either gravel or polished river rocks in the bottoms of my betta tanks depending on the look I want.
You can either use real plants or silk plants. A light would be necessary if you are going with real plants, but if you chose silk ones, it wouldn't. Also, depending on the warmth of your house, a heater may or may not be necessary. If your room temperature stays above 70 f., I wouldn't consider it necessary.
Also, if you are setting up a tank for multiple female bettas, try to get one large enough for a group of 5. I find they get along much better in a group that size. A 10 gallon would be perfect