You should firstly start with 2D sketching in blue pencil. The sheet below communicates some concept ideas for a Toaster. A toaster has been selected as most ideas would generally be based around a 'block' and this can help you to develop some good techniques before trying more 'organic' freeform sketching or 2 point perspective sketching.
The composition of ideas starts with showing 3 views of a concept idea. This is usually the Front view (as you look straight / head on at the product); a Side view and a Top view (what it looks like from above). Whilst these 3 views do not communicate all details about the product it can give a great starting point and help prepare an idea to be sketched in 3D.
Select one of your 2D design ideas and sketch your product concept from a variety of view points, including in 2 point perspective. Start this in blue pencil.
Add fine line pen (keeping it 'sketchy'). Add a little shading in fine line pen (0.3mm) to show where light may hit or areas of shadow.
You now need to render your design sheet. Remember that all edges are always dark and that you need a light source - either Top Left or Top Right.
Use of black soft pencils or Airbrush Shadows to add further depth and detailing.
Modelling in Foam