Flat Boot Fitting

When standing on your skis, on the snow, with your feet about a boot width apart, the skis should lay flat.  If they don't, you may have problems making your turns and gripping the snow with your edges.   Some boots like mine have a side cant adjustment (bottom picture left side) but for me, I need a lot more.
I have a problem being bowlegged and so, to flatten my boots, I have to add shims to my boot insert.  My shims are about 3/8" thick and made of rubber (1/4") and a couple of pieces of heavy felt. You can make them with just rubber.  I fasten them in place with duct tape and then push the insert carefully into the boot shell.  
Then I relocate the buckle catch on the top boot buckle to make room for the shims in the shell. Bottom picture.
The shims on my boots are located on the left side of the right boot insert and on the right side of the left boot insert.  
This is an easy, inexpensive, and very effective way to adjust your boots' soles to lay flat.
To check your boots, put them on and snug them up. Then stand on a hard flat floor.  
Start with you boots touching. With your one foot flat, only the outside edge of the other boot should touch the floor.
If not, you are knock-kneed and need shims on the outside side of your inserts. 
Or continue.
As you separate your feet about an inch at a time, you should reach a point where both boots sit flat on the floor, about a boot width apart.  This should be your normal stance on skis.  
If not, keep widening your stance until both feet are flat on the floor.  You  are probably bowlegged and need shims like mine, on your inserts at the insides of your legs.  
The best way to make a shim it to use rubber from an old tire inner tube (Tire Shop?).  Cut a number of squares to make layers. Then, by trial and error, add layers until your boots are flat at your normal stance.  Try stuffing the rubber shims in place before fixing them with duct tape. 
I know there are some companies that will mill a bevel on the bottom of your boots but this is expensive and you only get one chance to get it right.  A ski shop where you buy your boots should be able to help if you show up with all the parts.  
List: scissors, inner tube squares and the rest of the tube, contact cement, duct tape.