OK- lots of questions.. .let's see if I have plausible answers.
elverton- Use a medium to soft buttercream for the fill-in. I prefer a butter recipe. If you use an all-shortening recipe, use parchment paper instead of wax paper to do the BCT on.
I would still uses the outlines for the black sections. It just helps you stay within your areas and keeps other colors from bleeding into each other. No WalMart. huh? The bottom line is you need a thick/stiff icing for the lining. Make your buttercream as thick as you can stand it since you're not using a fudge icing. Once I used the Wilton black. It was too thick! I cut off the top of the tube it came in, I mixed it with my buttercream (just a hint of mine) I'm sure I needed to add a hint of coloring and used it. It was fine.
thecakemaker- My version of a glaze transfer is a but different than described by mari, although hers IS correct, also.
I print out my transfer picture. I go over the lines with wax paper. I turn this over onto my crusted iced cake and gently brush over it to adhere the piping gel to the cake. I remove the wax paper.
I go over outlines in black or whatever color I need.
Here's the glaze part: I take my buttercream icing (already tinted with color) and add equal part Karo or corn syrup to it. I add enough until it flows like honey. It should NOT keep it's shape when piped. I pipe this into the outlines. And like Mari said, you can play with it to get the feel. It also saves just as well as buttercream. I have some different colors I've had in storage for a couple of weeks I've used on several cakes. AND, it's nice and shiny!
I also use this method for writing, except I don't add as much corn syrup. Lettering, obviously, has to retain it's shape.
Here is the most recent cake I did with this method:Dash made with my glaze method
prettytoya- Yep, WalMart FUDGE icing works great for the lining. I've never used their other icing for anything. I make all my own icing with the exception of the fudge icing for BCTs and CTs.
If I missed any questions, let me know. Good luck!!