How can I finish my attic and make it a bedroom for my husbands workroom?

1st) Is it a real attic? ie) do you have rafters or trusses supporting the roof? Rafters you can work with. If its trussses, you are looking at an entirely new roof.

2) If its rafters, then frame it, run the electric, hang the drywall and paint it. Now add the trim.