When building almost anything for the dollhouse (windows, furniture or finish molding) making sure the pieces are square is absolutely essential.  Using some scrap lumber, you can build a jig that will be an invaluable tool in the workroom.

All you need are two pieces of 1 x 2 (6" - 8" long are ideal), a flat piece of plywood or MDF, preferably 3/8" or thicker, some wood glue and a good steel square.  Optional: several screws at least 1-1/4" long.

Look at the photos below and assemble your pieces.

Apply a thin layer of wood glue to the underside of each of the 1x2 scraps.  Lay them on the base wood (you will have a few minutes to work before the glue starts to set up.)  My base was already cut squarely -- yours should be, too.

Using the steel square, check the outside corner and make sure your scraps are at a 90° angle.

Check the inside to be sure it's also at a 90° angle. Wait for the glue to set up a bit.  Most good quality glues will dry enough for you to work in about 15 minutes.

For extra security, I screwed the scrap pieces down by countersinking wood screws.  In my shop, things are always falling off the workbench and this will keep your jig from snapping apart if it hits the hard floor.

That's it!  You can also apply a light coat of clear urethane or varnish to keep any excess glue from sticking.