how to make a router dovetail guide

[archimeech]

the tolerances required are very exact, much better to pay your bucks and get a commercial one. you can however make your own box joint jig

I agree with Kiwiles,

The time and precise fabrication required doesn't offset the savings. But if you really want to make one. Here's a basic exploded drawing of one. Then it depends on the depth and spacing you want to achieve.