"Heavy" wall shelves can mean one of two things: the shelves themselves are heavy or you intend to store heavy objects on the shelves. For truly heavy storage, your best bet is a free-standing shelf unit that is not built to rely on wall attachment. However, heavy shelves rated for hanging on walls can be installed similar to lighter shelves. However, following the steps for hanging them takes on greater importance to provide support.
Examine the mounting brackets for your shelves. Get one wood screw for every mounting hole.
Find studs in the wall where you want the shelf unit to be mounted. Mark the studs with a pencil at the height where you want the shelf to hang.
Set a level against the wall at the points where you want the shelves. Set it to level, then draw a line across the top with a pencil. Mark each point where the wall studs intersect your line.
Set up the two ladders, facing each other and closer together than the length of the shelf.
Lift up your shelf unit to where you want to hang it. Set each end on the steps of the opposing ladders.
Lift one edge of the shelf until it's even with the line drawn. Align a mounting bracket with a wall stud. Drive one screw through a screw hole in the mounting bracket.
Lift the opposite edge of the shelf and drive in a single screw through a mounting hole on that side.
Drive the remaining screws in through the remaining screw holes in the mounting brackets.
If you have assistants, you can do without the ladders and have them hold the shelf in place while you mount it.