A/C work can get expensive real fast.....tools cost and replacement parts cost.
IMHO....get a book.....it's a cheap investment.....then read it.
Proceed at your own risk. ALL LAWYER TYPE WARNINGS APPLY. Take precautions and wear safety equipment. Release of the refrigerant into the atmosphere is verboten.
DIY...OK....I'M NOT AN EXPERT (so, just assume that I‘m wrong).....get a book.....it's a cheap investment.....then read it.....a Ford is different from a Chevy. Last warning.
Short story.....first is it R12 or 134a (converted or factory)? Factories did the conversion to R-134a about 1994? So, at least you know what you’re dealing with and what type of refrigerant to get.
IMHO…..if you let the expert, dye it, or vacuum it……why not let him do everything to re-charge it?
OK……so, you still want to try? Last warning, again.
Check for leaks.....dye or evacuate (vacuum) the system and see if it will hold a vacuum (30min-60min, your choice). If it's leak free.....continue. IF not....you need to replace parts (don’t forget oil for the components) and/or tighten the connections properly.
If after you replace parts....check for leaks, again......why waste refrigerant?
Say you didn't test for leaks and want to gamble......assuming that the individual components are in good working order? And, you’re still willing to continue.
OK....engine off…….hook up the manifold. Consult the manifold’s manual (directions) on how to use it. Get the initial reading (AKA; Static reading). They'll be a point that the levels will equalize.....note gauges (hi and low side readings). Consult book. The reading will sort of tell you (though, not always)....IF, it just needs a little charge.
Assume here, that it needs a little charge.....ENGINE OFF......with the hoses properly connected and the manifold knobs turned correctly……..introduce the proper refrigerant and let it get sucked up through the low-side port. Can is right side up.
When it's though sucking up, as much as it will take.....note manifold readings. Close off all the manifolds knobs again. Consult the book. Test run.
Still NOT COLD?
Next, turn on the engine, set idle at 1500 RPM, A/C on, A/C fan running.....(a house fan in front of the condenser keeps things cool while the engine is running). Turn manifold knob to introduce more refrigerant into the system via the low-side. (Remember, there are specific capacities involved to properly do the job.) The can may need to be inverted and you mught have to jump the compressor switch (to help it suck up). Keep a watch of the high and low gauges and the temps that you are getting in the interior of the truck.......40 degrees (in F) below the ambient air temps is, "good enough,".......close off the refrigerant and disconnect hoses......It's done. Note the high and low pressures and compare with the book readings. Having an "over charge" is a bad situation too.
Warnings.........there are specific pressure/temps readings involved, and the book mentions that tests are at 80 degrees F ambient air temp. and your location (height above sea level).....so, you're to adjust to the book's table.
ALL LAWYER TYPE WARNINGS APPLY
Here is a good source of info on A/C parts and some other info (note, there is a forum):