Im at the point where ive got the decals and the wash on the model and have applied a flat finish. At what point do I starting adding chalk? And how do you seal the chalk to the model?

You can add pastels after a flat coat is on.

To seal the pastels - hmm - there are two schools of thought here .
1. - No sealer at all. Adding another coat will wet the pastels and reduce the 'dusty' look you're trying to achieve. Also to add airpressure to your model will blow off the loose pastels.

2. - Seal it - I've read that some guys use another light flat coat or light hair spray. Spray at a low pressure and let the flat coat 'fall' on the model.