Once you've worked out how long you want your shelves to be, you can have your timber pre-cut at your local Bunnings. We cut 3 lengths of 300m from the 185mm x 19mm timber and 3 lengths of 280mm from the 42mm x 19mm timber.
Mark where you're going to drill the 3 holes in the bottom of the shelf support. Make sure the holes are in the middle of the timber and an equal distance apart. Then clamp the shelf and the shelf support to a workbench so that their edges are flush and pre-drill the 3 holes.
Apply some PVA wood glue to the back of the shelf support. Then clamp the shelf and shelf support to the workbench again. Drill three screws into the holes you've already made, driving them below the surface of the timber with a countersink bit.
Use a putty knife and putty to cover up the screws in the shelf support and to hide any gaps between the joints of the timber. Wait for the putty to dry, then sand it off until it's smooth. You can also give the whole shelf a sand for a nice, even finish.
Giving the shelf a stain or paint before you hang it will make it look great. If you're staining it, for a more professional look, wait for the first coat to dry, sand it back lightly and then apply the second coat.
Now you need to choose where you want the shelf to be. Using a spirit level, mark on the wall the distance of the holes you drilled in the shelf support. Screw wall anchors into these marks.
Line up the holes in the shelf support with the wall anchors. Then drill screws into the wall anchors to secure the shelf.
You can also putty up the holes, sand it back and apply more stain for the perfect finish.