I first do very rough ink sketches to work out the shape; none of these are exactly the finished version, but they help to pull the ideas in my head into focus.The next step is to build the plane in a 3D modelling program; if this seems like cheating, the process was inspired by the working method developed by Frank Hampson's team on Dan Dare. They would build detailed models and photograph them; I make rough models in CGI and work from those.
Most of the elements (wings, engines & fuselage) are taken from free models supplied with the program; I rearrange, scale and distort them to get the look I want, which is much quicker than trying to build from scratch, especially for a newbie modeller like me. The chequer pattern on the plane (and on the slabs in the background) is to help with drawing in additional detail in perspective.
This is the half-finished drawing; the plane has been inked in, and the other elements roughly drawn in following the perspective from the CGI rendering.Details like cockpit windows are much easier to draw by hand than to model.