What conditions do you give them?

They need to be out of the sun but in good light and they need very good humidity. They also hate being dried out.

Ferns aren't that easy to grow indoors - they hate dry air, so a centrally heated home isn't an ideal spot for them. They also will not grow in the presence of woodsmoke from an open fire. That said, if you can find a spot in the home which is away from a direct heat source such as a radiator, which has a reasonable degree of humidity (bathroom, kitchen) its worth selecting one of the 2,000 varieties available. Easiest to grow in the home are: Cyrtomium, Davallia, Pteris cretica, Nephrolepsis, Asplenium nidus and Peltaea rotundifolia. If you have somewhere you can fix a hanging pot or basket, then Nephrolepsis and Adiantum make good hanging subjects.