Well now, after reading this thread I now know why you people have so much a/c trouble.
The lowside port is on TOP of the accumulator which is under the passenger fender, you do NOT have to take off any parts to get to it. You MUST however have long enough charging hose to allow you to work on it without laying down.
NEVER charge through the highside. Only an idiot does that. I know someone who lost 3 fingers doing so. The can has 70-80 psi in it and the highside can easily be 300 psi, what do you think the can will do? Short answer=explode.
If you would put a highside gauge on instead of thinking you are doing it 'correctly', you'd see what happens when you turn the can upside down to get liquid, the gauge goes spastic with uncontrolled pressure spikes, good way to tear up your compressor.
You cannot properly charge a system using just the one lowside gauge, thinking so just makes you look silly. They make kits like that to keep from getting sued into bankruptcy, seeing the responses here it becomes crystal clear they made the right decision. Same reason for the highside having different bigger size fittings. To protect you from yourself.
The motor needs to be running while charging so active low pressure suction helps you to evacuate the can.
If you have suddenly lost your cooling, please get a check for leaks in the system before you recharge it. You may very likely save a bundle doing so, most times you have sprung a leak. I used to watch all day long people pour that stuff in to the tune of $60-$70 at a time before they came to grips with the fact it was all leaking right back out.
Don't overfill thinking you are making it better, any coolant beyond what system needs just makes the cooling drop right back off. Of course you WILL be overfilling since the one gauge method does not clearly indicate when to stop charging like 2 gauges will.
Look here. The lowside will show the same 25-50 sweep with the highside at say 150 that it will with the highside at 250. See why you can't use the lowside to properly fill? The HIGHSIDE determines whether system is full.
When the system is totally empty you must charge it up to at least around 25 psi before the clutch comes on and starts cycling.
Here's hoping you vacuumed the thing down, any air left in carries water that freezes across the restriction to make it stop and go cool. Works for a minute then stops, starts again when the ice melts only to freeze again. You can make a cheap pump out of a small room a/c unit that still has a good compressor in it. What I've used for years, while not ideal like a $400 pump, it works good enough and cost me nothing. It can be used for free leak checks too.
The lowside reads from 25-50 give or take a few. It needs to sweep up and down through that range. At 25 compressor comes on, 50 off. Doesn't have to be exact just close. The highside needs to read what the freon temp chart says for the day's temp. I do mine a lot past 90 degrees and usually end up with anywhere from 275-300+. I find that past 90 degrees, if you closely watch compressor, it will stop cycling off/on and start to stay on all the time at idle, I go slightly past that point, usually the highside will be about right. It MUST cycle on/off though when the engine speed is raised up to like 2000-2500 or so.