Ammonium chloride comes in powder form. I bought it on-line from walmart. Its water soluble so I just mixed it with water in a jar to the consistency of paste and used a paint brush to apply it to the kettle.
I got the tin on-line from roto-metals. They have bags of tin pellets. One pellet goes a long way.
I got some liquid flux from the hardware and put it in an old windex bottle so that I could spray it on areas that needed a little extra fluxing during the process and also to dampen the piece of fiberglass insulation I used to spread the molten tin around.
If you do a search on youtube for tinning brass, you will find a bunch of stuff.
My first attempt I didnt clean properly , didnt use the flux and used a piece of cotton as a wiper. The tin wouldn’t spread, the cotton burned and it was basically a big fail. My second attempt using the method described above went pretty easily