Pump up the tube to find the hole. Let nearly all the air out of the tube again, clean and dry the tube around the hole, and use the sandpaper to roughen the area. File off seam protrusions. Apply a thin coat of glue that is larger than the patch. Apply the patch and press on hard at the center and all around the edges.
Continental patches seem to be the best.
To reinstall the tube, inflate it so it just barely holds its shape. Insert the valve and push the tube into the tire there. Working away from the valve, push the tube into the tire while slightly pushing it towards the valve. Alternatingly work from the left and right of the valve to prevent the valve from being pulled in one direction. If the tube is so long that a loop remains after it is fully installed, don't squeeze it in but pull it all the way out again and retry, this time pushing it harder towards the valve.