Ok I wanted to do this just in case we had some new builders out there that may need a little help, like me... I learned this the hard way trial and error, there is most likely something already out there on the subject..
Add your foundation..
Then your wall..
Ok turn your cheat on constrainfloorelevation false, and add your garage wall.
Now get your level tool out, and level..
Now draw your walls back, on the 3 missing sides..
Now delete the bottom tiles, where you want your garage door to sit..
Now Only Draw Back The Tiles You Deleted, NO MORE...
STOP before you go all the way to the end..
Ok get your level tool out again this will be the last level..
Level from the top this time...
Now Add Garage Door...
Now add your siding, I use the Vertical Slats because the line better and hide the flaws of the slant, and you can paint them any color...
and there you go it's done... On the inside you can use buydebug lights and set them, an the reflect the shadow and hide the flaw also, you just have to play with it. Well this ends my tutorial I hope it helps...