O1. Find the brushes you would like to download, and download them. They will most likely be in .zip form.
O2. Open your files. Right click on the .zip folder containing the brushes, and select "Extract All". Click OK.
O3. Move the extracted files to GIMP's brush folder. On a PC, this can be done by moving the files from:
Computer >> Local Disk (C:) >> Users >> [Select your Username] >> .gimp-2.6 >> brushes
If GIMP was open while you were downloading the brushes, you'll need to click the "Refresh" button at the brushes box. If that doesn't work, you might need to restart GIMP. Layers
To keep all layers visible, make sure that in the Layers box, the eye icon next to all the layer names are there. If it isn't there, click one of the boxes next to the layer names and the eye icon should pop up eventually.
While you can view all layers at one time, you cannot be on more than one layer at a time.
Hope that helped. ^^