PCB prep: I sand with 220 grit and clean with 90% + Isopropyl and Bounty paper towels. I also clean my fingers with the alcohol to prevent re-contamination of the boards with finger prints.
Apply toner transfer, apply foil transfer, etch, sand again (lightly) with 220 grit to clean the copper and wipe down with acetone to remove toner remnants.
If I am doing double sided boards there are additional steps to place registration pin holes for aligning both sides of the toner transfer, but the toner transfer and foil transfer are applied to both sides simultaneously.
This project has proven invaluable in perfect (toner & foil) transfers every time:
https://hackaday.io/project/3363-apache-al13p-laminator-one-pass-pcb-toner-xfer
I also use a Full Spectrum 5th gen, 40W Hobby laser to make reusable solder stencils from stock 3 mil laminator sheets (comes with the Apache laminator). Simply export the (top or bottom) Cream layer from EAGLECAD and raster cut at 50% pwr & 50% speed.
The stencil for the board shown before took about 5 mins to make and about 5 mins to apply the solder paste. Clean the stencil with laundry detergent and a dish sponge.
Here are some more pics of the progress: This is another PCB from the same project.
Applying paste via laminate sheet stencil:
Paste results:
Assembly in progress, FR1 board so the 300°C hot air 'browns' it a bit: Only took 10 mins to solder, faster than using the solder oven.