I needed to make 2 small half mortises for a project. I was thinking this would be the first real task for my trim router. Each time I went out to the shed, I looked at that and imagined how I'd have to create a template and all the ways it could go wrong. So I put it aside for months.
I picked up this project on Australia Day. With fresh eyes I realised all I needed, is to chisel it out. Took me about 10min each and both are basically perfect fits.

Another simple lesson for this simple beginner. KISS.