Oh, dear! I'm made a mistake and I need to use the mask to fix it...but how?? I left the 'sketch' with the outline on the same layer. Any suggestion?

well, i remember commenting about using masks before, but i'm too lazy to find it.

basically mask allows you to either edit only a specific color, or edit everything but a specific color. for instance, if you mask on black (a sketch for instance) you can either color just on the sketch, or you can leave the sketch completely in tact and color around it.

the easiest way to set up your mask is to right click on your picture (in lascaux sketch at least) to choose a color, then to right click on the mask bar to set the mask color, which you can change if you left click on it. you can turn it to off, normal, and reverse. this is generally how mask will work in the programs, but this example is only based on how lascaux sketch is set up.

hope this helps you out, have fun messing with masks (very powerful tool aside from layers).