[header] How to install mail slots in doors [title] Purchase your mail slot. [step] Buy the largest one that will fit on your door to accommodate larger envelopes and magazines. Another good idea is buying one with a basket to catch the mail as it comes through the door slot rather than allowing it to scatter across your entryway. 
[title] Stand in front of your door and determine the best height in which to install the mail slot. [step] Make a pencil mark at that height to use when you begin laying out the template.