You did say a chicken house right?
My experience:
Pondering:
Find suitable window (I have a bunch of "pulls" from a house remodel, they had moisture between one of the panes). Cost $0
Find where you are going to put it/them. I had a metal angle iron running at waist height I had to go above.
Study a couple of window frame cross sections - pay attention to drip edges.
Find suitable scrap 2x4's.
Piddling/Puttering:
Build frame, cut hole. Put in drip edge.
I've a BUNCH of pieces of Corian sink backsplash pieces about 6" wide. They will NEVER need painting. Excellent drip edge, I just shimmed it to ensure a slight angle so water runs off outside.
Install window.
Seal w/ appropriate flexible sealant - This was the biggest cost, must have spent $10 on sealant and screws.
Put in a little spray foam above & below window in the "bumps" (ridges) in the siding. May not be needed for a chicken house.
BIGGEST problem - deciding on tool(s) to cut siding. Best for me is a combo of HF 14 gauge 4 Amp Heavy Duty Metal Shears and manual tin snips for the bends where the HF's wouldn't do. I tried circular saw, reciprocating saw, jig saw, metal nibbler, just snips, angle grinder and that combo works best for me. Couldn't justify a plasma cutter

Yet.
/edit - this was for the apartment in my 40x60 workshop.