Deck belt is 1/2" wide 79" long. Tractor Supply has them pretty much every store.
I'm not sure of the size but if you look under the hood mine tells me the part number. Take it to any parts store and they can cross reference one for you that will fit.
A Craftsman 42 deck belt could be smoking because it is rubbing against something. There could also be an oil leak. If you can not find the cause, you should take it to be serviced.
31''
3/8 x 33.1"
1/8" feed line 3/16" return line.
Usually there is a problem with the belts that drive the blades, after you stop the mower. Take a close look at the belts and assure that the belt is tracking and the pulleys are aligned. If you are unsure of the belt routing recheck the manual.
If you are just replacing a belt then take the old one to an auto parts store, they will size it for you.
The first link will take you to the manual and pages 22 and 24 should help. additionally the second link will take you to one of my sites and it is the EHP decks that will be of interest. I encourage you to check both; the second on because I separate the belts and routing. I am assuming the deck belt.
forward drive belt is a Gates # 6838 38x1/2 reverse belt is a Gates # 45x1/2
37x63
Most LT12H mowers take a #213676 OEM belt or #6885 Gates belt for the blade application
The classes that someone needs to take to become a craftsman are college classes. They would need a vocational certificate in crafting. This of course only applies to professional craftsman. No classes are needed for freelance work.