I learned the principles of how to do frame line painting when I was learning how to build frames at Ellis Briggs Cycles in Shipley West Yorkshire in 1975. Here is a picture of frames made and painted with double box lining at EB (only the 2 on the left end) after the war.
The 2 frames on the left are original paint from when the frames were made in the early 50's.
Their frame building room was next to the painting room. I didn't try my hand at spraying while I was there but it was a huge advantage to me to get to watch what they did so i knew what to do later. Rod one of their painters used a sword brush. He would hold the brush with his thumb and forefinger and would guide the brush along the tube with his middle finger. This kept him from wavering. He would start sooner and go longer than the final result. After he did the cross stripes, he would wipe off the excess beyond the corners. Here is a picture of a Johnny Berry he painted while I was there.
Rod painted this 1953 Johnny Berry frame at Ellis Briggs in 1975
When I started to make and paint frames in Niles, Michigan after I got back from England in 1976, double box lining was considered to be old fashioned and nobody ever requested it until it became popular again about a dozen years ago.
When I made my daughter a custom frame a few years ago I wanted it to have double box lining. I used the same method as described by Mr.66 in post #17. I chose to have one box a white pearl and the other box a yellow pearl. The frame itself was painted in 2 shades of gray pearl. This was to coordinate with the Dura Ace gray. Here are some pictures of her bicycle:
One of my colleagues uses a local sign painter to do his double box lining.