OK Jgrden,
I think as Seabronc said you may want to leave this to the pros...however I applaud your willingness to try this on your own.
First off use the proper safety equipment. Safety glasses are essential. I would be without one eye if I hadn't had a pair on when working on one old vehicle.
Gloves are needed too. Your gauges (or the neighbors) should be in good shape with no cracked hoses or broken parts.
The vacuum pump you purchase MUST be able to vacuum an AC system to a minimum 28 in/ hg.
Connect the gauge set with the center ( yellow) hose in a metal can or plastic bottle.
Engine off of course, Open the low side valve slowly and allow the Freon to escape at a slow pace. Be patient. When the hissing is appeared to be stopped, open the high side valve and allow any residual freon to escape.
You may now proceed to remove the compressor. ( sorry there isn't any drain.) turn it with the port side down in a pan to collect the oil.
Important! save the oil and measure what came out. Compare this with what is the normal capacity of your type of compressor. ( sorry I don't have that info, I'm sure someone on here does.)
Now you have to examine all the lines and disconnect them where there is any connections. ( This may require special tools to unlock the fittings.)
Remove the expansion valve and leave it out to help allow flushing the evaporator. (replace it with a new one.)
After all the lines are disconnected you can use the flushing fluid ( probably will need 2 bottles) to clean out each line and component. Put a rag at the outlet point of each component you flush to catch oil, and debris.
Do NOT flush the compressor.
Do not flush the receiver drier ( accumulator) get a new one.
Allow the cleaning fluid a little time to dry.
Replace the oil in each component with new ester oil. 2oz in the evaporator, accumulator and condenser and about 1oz in each line.
Be sure to have enough oil to fill the compressor to the proper capacity.
**** all the new O-rings with ester oil and install each component and connect all the lines. Remember to install a new expansion valve.
Install the compressor with the correct oil. ( It can be a bear to get it in place without loosing oil. A little loss is ok..alot and you need to re-measure and start over.
Once the system is all back together, allow some time for the oil to run into the reservoir at the bottom of the compressor. 10 min should be good.
Hook the gauges back up and connect to the vac pump. Vacuum for a min 20 min. I prefer 30 here in Fla. where it's humid. be sure you have both low and high side ports open. ( some vapor from the vac is normal for the first few min.)
The vacuum should reach at least 28 in/hg and stay there.
After the vacuum time, close both valves while the pump is on and effectively seal in a vacuum.
Look at the gauges.....wait for 10-20 min. (yeah 30 is even better.) to see if the vacuum held. If so your good! If not, you have a leak,
( . Got to trace that down. Better going to the pros that have the proper leak detection equipment.
If your vacuum holds, you can tap a can of R134, purge the line to the gauge manifold and add Freon to the low side only. Most of the first can will be sucked in by the vacuum.
Start the engine. Set idle around 1800 rpm. turn the air on max, the fan on high speed.
Follow this procedure to add additional Freon to the spec for your truck. (As Al said above about 70% of R-12 capacity.)
As you add Freon the compressor should recycle less frequently. ( you should not have to jump the LPS, but may have to initially)
Watch your gauges. Low side should be between 15 and 40 and high no higher than 300.
This should get you good cold air, and a reliable system.
Now remember, you need to buy the Freon, the flushing fluid, an accumulator, expansion valve, ester oil, and O-rings...and do all this work...and you are your own warranty.
Good Luck