Hoo boy! Where do you start? It all depends on how dedicated you are and how much time you are willing to spend. First off, All grease has to come off or the paint won't stick for long. Second is the rust. If you don't get it off it will continue under the paint until it eventually flakes. A good steam cleaner with a degreaser works well. I have also used chemical strippers with good success.
As far as painting is concerned, any items such as hoses, Plug wires, fuel sediment bowl, distributor cap, steering wheel, gauges, nameplates, shift knobs, fuel and oil filters, and the list goes on, should either be taped with masking tape or better yet, removed. If you want to do a decent job, all sheet metal  and tires should be removed so that the rest of the tractor can be painted. Unless you want the radiator green, better remove that too....and the V-belt. I hang a lot of the removed pieces all over the shop and paint these separately, including all sheet metal.
I find the rims are the biggest pain in the butt, it takes a lot of paper and masking tape to cover the tires adequately. Newspaper is fine.  I have seen a lot of nice paint jobs that have overspray on other parts such as tires, starters,hoses, etc.. It is my opinion that a moment of laziness makes a poor job of what would otherwise be a quite nice job. If you intend to keep the original decals, you must tape them off with masking tape. There is some new types of tape that have only a little of the "sticky" on the back side so that they don't lift anything that you applied it to.  I don't remove gauges, but I do remove shift knobs,lights,mufflers, and  pieces of a different color than what I am spraying.(when possible)
  Don't try to spray it all in one coat. It is better to give it multiple light coats and have a lot of missed spots which you will have to go back for than try to lay it on too heavy only to get runs. I have found that if you do get a run, wipe it immediately with a good cloth, wait a few minutes, and then give it a respray.
   Probably the most important thing to have is good lighting....you can't paint in the dark. I have at least 4 halogen lights, and move at least one all the time to flood the immediate work area.
  Be sure there is no dust on the tractor or shop floor before starting. (if anyone opens a door and lets the wind come thru, it will kick the dust up... and you know where it will go!)
Make sure the shop, the tractor, and the paint are warm. Cold paint won't lay down well. (Long ago I painted a pickup when it was maybe 50 degrees and the paint ended up looking sort of like an orange peel.)
 I have a pretty good book on tractor restoration...written about farmalls...(no, I don't have any!), but it sure has a lot of good info regardless of what tractor you are working on. If I can find it, I will let you know what it is. They have a pretty good chapter on painting.
   Have a good quality paint sprayer and air dryers on the air line to prevent moisture from entering the sprayer sure helps too.
 Well, that is it in a nutshell, perhaps others can add their two cents also. I am sure there others who know more about this than I.
          Hope this helps,
                       Cal