when I used to trim cars we used an electric kettle :lol:
this works with suspended fabric roof linings only ,Fill the kettle and tape the switch down then put it in the car close the doors and turn on the power and leave it to fill with steam . Keep an eye on the water level and top up as needed .. the idea is to steam the whole roof in one go so it shrinks evenly taking out all the wrinkles.
however it could be that you have wet the roof and shrunk it in places pulling it from the roof sheet.
in which case don't shrink it any more . The only good way to fix this is to take the lining board out carfully peel the lining off , size the board (made of compressed dry fibre) with spray glue let it dry then re stick the lining using 3M (77 spec) spray glue. It is however often easier to buy new head lining @ about £6 mtr from a trimmers warehouse like
http://www.boyriven.co.uk/