Step 1: Decide if you have the necessary skills and expertise to undertake ALL the project steps. If you do write up a plan with all your steps and how long you think they are going to take.
Step 2: Inspect the truck completely. Figure out every little thing you can that will need replaced/repaired etc. Without hurting your resale value (as much as is feasible) by stripping it right down in case you decide to abandon ship and sell.
Step 3: Do a compression test and see where your motor stands.
Step 4: Do a budget:
-Figure out how much you need and where the money is coming from.
-Figure out how much TIME you need to do the project..and if you have it available
-Decide which items you can't do or don't want to do..and get quotes on them from local shops.
Step 5: Take all of the above into account and decide if it's the right time to undertake your project.
-If you have time but no money...ripping the truck apart is free
-If you have time and little money....maybe you run the truck as is and fix stuff as you go
-If you have time and some money...pick your big ticket items that are critical to being on the road and get those going...save all the little details for the end in case money gets tight (eg fancy rims)
-If you have lots of time and lots of money...dive in head first. Don't be afraid to hire a pro to help out with the hard stuff.
-If you have no time and lots of money: pay someone to restore the truck as you see fit.
-If you have no time and no money...don't do it! (wait for a better time!)
Step 6: Doing the actual project:
If you decide you are ready to actually take it all on...I usually start by stripping everything completely down. Decide if you're blasting and get the frame blasted and painted.
I like to have something rolling that I can put things on...it's a lot easier to move
Running your lines with your frame nice and clean is pretty sweet...as long as you know what goes where...
Personally I'd start poking around the cab and figure out what sheet metal I need to fix/replace.
I would decide what you're doing for your drivetrain and at least get all your brackets, mounts, etc figured out. It's also pretty nice to have the whole drivetrain on there before the front clip and cab are in your way.
Really depends what you have for parts/money too....if you're waiting for an engine rebuild why not plug away on the cab for example?
Mocking things up is never a bad idea...just a time consuming sometimes...
Sorry this got long winded.
Houses get built from the foundation up....I like to do trucks the same way.