Reddit mentions of CNBTR 1/4 Inch Shank Dia Silver & Red Carbide Woodworking Straight Router Bit Set Pack of 7

to expand on this.

you'll want to buy a cheap powered router, a straight bit, a couple of clamps and HEARING and EYE PROTECTION.
i can't stress those last two enough.

once you've done that,

1. you'll want to install a wide-ish straight bit into your router.
   
   
2. mark off the area you want to route (make the marks on the waste side, where you're cutting away, or the other side, but be consistent).
   
3. set the router so the bit won't cut, but you can see it in the flat shoe on the bottom. you want to line the edge of the bit to your line, then mark where the edge of the router is.
   
4. you want to clamp your metal brackets to that line as a straight edge and guide.
   
5. don your eye and ear protection. it's going to get loud.
   
6. take a ruler or use the built in guides to set the bit to the depth you want (1/8")
   
7. double check your clamps are tight and the edge guide is straight. measure both sides of it, don't eyeball.
   
8. place the router on the work with the bit hanging off the edge. then turn it on while steadying it with your hands.
   
9. cut along the guide and any remaining bits from the edge until you have a nice level area for your bracket.
   
   i recommend practicing on a bit of extra wood. get the idea down and then do it on your final piece.