How do you finish a window installation?
google "trim interior window" = 

video:
http://www.thisoldhouse.com/toh/video/0,,20046338,00.html

steps:
http://www.thisoldhouse.com/toh/how-to/intro/0,,218130,00.html

Assuming you have already:
1) insulated in the rough opening with low-expansion spray foam
2) sheetrocked to the rough opening