If you aren't going to do it right...
Sorry. Windows are a {explicative}. If you are going to do it fast and dirty, get some good tape whose appearance is not to offensive to you.
Black butyl sealing tape or the
foil backed stuff. Wrap the tape around the edges of your window cut out - that will protect them from moisture intrusion. Slip in your window, and nail up the flanges. Now tape again, up both sides, THEN the top, so it overlaps the top edge of the tape on the sides. No tape on the bottom. Its ugly, but it works - and it leaves a straight edge.
Now cover with trim if desired. The whole idea is to keep water on the "outside" and give a way for water that gets in to get back out. By ensuring that your overlaps are topwards of one another, you greatly reduce the chances that water rushing downhill will get between/under layers.