Here is a little tuto about my use of pastels.
First, the pastels are used at the very end of the build: aircraft is finished, matt varnish airbrushed.
I use different kind of pastels: my preferred ones are from Tamiya but I use also pigments from MiG (essentially for smoke effects) and, for the nuances Tamiya don't have, some chalks boughts in specialized shops. Those ones must be reduced in powder to be used.
I put a paintbrush (size depends of the surface size of the model) into the pastel choosen, then I remove the excess of pastel on a sheet of soft paper. I apply gently the pastel on the surface (each panel is treated separately; that can take some time to finish of course...), starting from the center of the panel and going near the extremities - but taking care not to cover ALL the panel.
To be precized that if the panel is covered by two different colors of paint, each color is treated separately too.
When finished you can wash the paintbrushes with water. Use different paintbrushes for the different colors because if the paintbrush is wet you will not obtain a good result.