Most of the materials used in my illustrations come (regrettably) from the coastline close to my home.  
A fair amount are found on our streets, in parks, green areas and other public places, as well as from household recycling and other odds and ends.
All of these bits and pieces are thoroughly cleaned, dried and then stored in large containers (pictured.)

Occasionally, a found object will inspire a design straight away, its physical appearance immediately reminiscent of a spacecraft, or a probe, or a two-headed robot, or some other weird cosmic machine - but this isn't always the case, and much of the time I will already have an idea for a finished piece in mind, and a lot of what I collect is based around it having "future potential..."

I usually sketch out initial ideas in pencil first, letting my imagination run with a concept and exploring as many design alternatives as I can imagine.

After this stage, it becomes a matter of trawling through the "bits box" for parts, parts that best lend themselves to the sketchbook version of the piece.

There is invariably a compromise between the materials to hand and the original design.
This part really makes the build more interesting for me, and the resulting piece becomes one that has evolved almost independently - beyond complete control - it can be quite surprising to see what turns a project will take, and it is one of my favourite parts of the process.