Hi, I'm not sure what you are asking.
Do you want to put the lines on one layer and the coloured area on another?
Do you need information on what approach to take, which tools to use, or where to find those tools in the GIMP UI and how to use them?
"Klinger, do you know how many zoots were killed to make that one suit?" — BJ Hunnicutt, 4077 M*A*S*H