1. Take a piece of plain white box cardboard. Something like an iphone box cardboard will work.

2. Cut it to the dimesions of 50x50 mm. Can be smaller depending on your mouth width.

(Dry your mouth of any saliva until it is very very dry)

3. Put the piece of cardboard against your upper teeth and align it straight with your dental arch and hold it there with your index fingers. Align it in a way so there is an equal width of carboard on the outside of molars of either side on the back, as well as equal length of cardboard behind your last molars and your front incisors.

4. Now use your thumbs to push the cardboard against the teeth of the upper arch. Start at the back and work your way to the front. Do this 2-3 times really really hard so you can have clear impressions.

5. Take the cardboard out and dot the impressions with a pencil. Draw a line connecting the dots on both the inside and the outside to get the shape of your arch.

6. Draw a straight line between the inner edges of the first molars. Measure the length of the line.

7. Subtract 1 mm from the final result to account for the gums.(Since, as described by Mike Mew, IM width is taken from the most gingival points on the inside of 1st molars.

8. The final number is your intermolar width.

9. You can also measure your palatal length from the same impressions, to track changes in length. To do so, draw a straigth line across the back borders of the last molars and the front borders of the front incisors. Draw a straight line connecting these two lines perpendicularly and measure the length of the line. This is your palatal length.

9. Keep the impressions for future reference. Write the Date and results on the back of the cardboard. Take these measurements about every 6 months.

10. Done!