I posted this on a similar post a while back, but here's my basic progression:
1. Wash parts while they're on the trees.
2. Cut every part off the trees one by one, trimming off excess and sanding mold lines and the like as I go.
3. After sanding a part, I put it in a plastic bag labled with the car and color that the parts need to be painted. I usually have a bag for flat black, gloss black, body color, interior color, metals, and other.
4. I put together like-colored subassemblies (engine, frame and suspension, etc) and sand off the glue and seam lines.
5. I prime everything by color (to keep it organized) and spray bomb the flat and gloss black parts.
6. I hand paint the stuff that needs to be hand painted. This includes blackwashing the grill and anything else that needs it.
7. I assemble and detail the rolling chassis, engine, and interior.
8. I paint the body, pretty it up with wet sanding, clear it, and pretty that up, too.
9. Install glass.
10. Finish assembly.
Each of these steps is also liable to have something like:
3b. Accidentally break something.
3c. Cry.
3d. Raid parts box to find replacement.
or
8b. Mess up paint.
8c. Cry.
8d. Purple pond.
8e. Goto 8b. Repeat as necessary.
Due to these unforseen circumstances, I sometimes will start by prepping and painting a body. If that goes well, then I continue. If it doesn't, I'm not as likely to get burnt out and relegate the model to the "started, but unfinished" stack, because I haven't put a ton of time into everything else.