We will firstly start with 2D sketching in blue pencil and in this case the sheet below communicates some 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.
The composition of ideas starts with showing 3 views of an 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.
Below are a range of of 2D sketches of ideas for a toaster.
There are 3 views for each idea (Top, Front, Side). Note that the sketch has the 'floor' or 'surface' for the Front and Side views.
This idea uses a number of 2D views (Top, Side, Front, End) alongside some 3D sketches to try to communicate as much about the design idea purely in sketch form. This could be extended by adding a foam model instead of the 3D sketch.
Added fine line pen (keeping it 'sketchy'). Added a little shading in fine len pen (0.3) to show where light may hit or areas of shadow.
A marker rendered version of the design idea (no annotation). Follow the advice for marker rendering. Remember that all edges are always dark and that you need a light source - either Top Left or Top Right.
Use of black soft pencil to add further depth and detailing.
Modelling in Foam