I've liked flames since I was a little kid growing up in Southern California during the '50s. I can't remember if the first time I ever saw something flamed was in one of my dad's custom car magazines, or on a hot-rod driving down the street. I do remember by the time I was in elementary school I was getting in trouble with my teachers for drawing flames on all of my school papers. When old people used to ask me what I wanted to be when I grew up, I told them a custom painter. By the time I was 14 I was custom painting all of my friend's bicycles in the neighborhood. By age 17, I had opened up a custom paint shop out of my parent's garage.
Through the years I've learned there are numerous different ways to paint flames, and that the best technique to do them with is whichever works best for each individual. Some novice painters might find one particular method hard to grasp, while another technique might be as easy as pie for them. The first technique I experimented with way back when I was initially teaching myself to paint flames was to entirely cover the surface with masking tape, and then draw flames onto tape. The next step was to cut out the centers of the flames with a razor blade and then spray-paint the exposed area. At the time this wasn't a very good way to go because it consumed an enormous number of tape rolls. In recent years a mild adhesive-backed masking paper has been introduced in wide widths that now makes short work of creating a stencil for flames.
The next layout method I discovered is the one that I continued to use for the entirety of my professional custom painting career, and it's been a staple for many custom painters as well. The first step is to lay out the flames with either 1/8-inch, or 1/4-inch 3M crepe masking tape. The choice of width is a matter of personal preference. I prefer to use 1/4-inch because it holds a straighter line, and I can turn it as tight as 1/8-inch tape if the need arises. On the other hand some custom painters prefer 1/8-inch tape because it follows tight corners easier, and line straightness doesn't seem to be an issue. Back in the day when I started custom painting there was only one type of tape: crepe (as in crepe paper). Since custom painters are judged on craftsmanship as well as design ability, one of the tricks to using crepe masking tape is acquiring the skill to lay it down so that the paint doesn't bleed under it. This paint bleed creates a fuzzy edge known as "creep." The later type of masking tape available on the market today is made from plastic and is called 3M Blue Fine Line. It's known for its ability to produce a nicely defined sharp edge (line) for even the most inexperienced painters.
One of the things a person will discover as they become more familiar with custom painting and its application is that various brands and types of materials all have unique working characteristics. Which brands and materials a custom painter devotes his loyalty to is decided by personal experience and preference.