Buy the cold bluing kit here: https://amzn.to/2PGwQH4
Phillips Professional Cold Blue: https://amzn.to/2wSM8BF
This is the entire process from start to finish on using a gun blue kit. The kit that I used can be found on Amazon Prime for around $30 shipped. This kit produces very nice results as long as you follow the directions and keep the surfaces very clean.