I'm the type of person who writes down what I do and how to do it in case I forget or need to go back and check. I have to qualify my thoughts with the particulars of my smoker - it was a reverse flow horizontal with a grease trench down the middle of the RF plate. The grease trench led to a grease drain pipe with a 2" 1/4 turn ball valve. Cooking, I left that valve open all the time. On a big cook I would sometimes get over 2 gallons of grease coming out that pipe. I learned to go up to the paint store and buy their paper containers that they sell for people to use for one-off paint holders. I would drain the grease into those and then put it into my freezer. The morning our yard waste gets picked up I'd put the frozen grease, still in its paper bucket, in our yard waste bin. (Where I live you can put fats into the yard waste since they use sophisticated composting techniques that can handle meat scraps and fats.) That's one aspect of cleanup that I do during the cook.
After the cook I let the cooker get all the way cold. The first thing I'd do is to open the firebox lid and use my shop vac to vacuum out the ashes. I used one of the extensions and that did a good job for me. I never had any problem with my firebox rusting.
The next thing I'd do is to take out the cooking grates, set them down in the grass of our yard, and pressure wash them with a small electric pressure washer I bought used for $15. That got all the heavy chunks and grease buildup off and left just a super thin sheen of grease which kept the racks from rusting.
When I built that cooker, I bought a common garden hoe and heated the neck to red heat and straightened it out, turning it from a hoe into a long-handled scraper. I cut the handle in half. I used this tool to scrape the cooked-on residues on the RF plate loose. After all the chunks were free, I would again pressure wash, this time the RF plate. That cooker also had another 2" pipe with ball valve which was the washout drain. I would have both the grease drain and washout drain wide open while pressure washing.
I built a log lighter (propane burner) in the firebox of that pit. After all the cleaning, I'd run a small propane fire for 20 minutes or so just to get things dry.
That is my method. That cooker never smelled rancid in the 3 years I owned it. Always sweet.
seattlepitboss
