1. You have save the brushes you want to your computer, after you have saved open up your documents and go to the saved brushes ;which need extracting first, right click and 'extract files'. save the exracted files and open up the extracted brushes.
then open up your GIMP folder , mine is in the C:drive under document and settings. Go to gimp then 'brushes'
so you should have TWO document windows up, your extracted brushes and GIMP brushes.
Then select all your exracted brushes , copy them and go to GIMP brushes and paste. next time re-start GIMP they will be there.
2. First, paste the lineart then go to colour >> then 'colour to alpha'
your lineart is now on a transparent layer. call this layer 'lines'
then make a new layer called background, put this behind the 'lines' by pressing CTRL + L and simply drag the background to the bottom.
then create a transparent layer called 'the coat' and have this layer inbetween 'background' and lines. This is your middle layer. if this doesn't work then just press CTRL + L to re-order your layers to your style.
there are probably better ways of doing number 2 but you'll to wait till an expert posts