How to recharge a window unit air conditioner?
It's a waste of money to try.  Especially if you're the guy who pays for the electricity.  

Captain Kirk didn't beam the Freon out.

First, you'll want to fix your Freon leak.   After all, air conditioners are generally sealed units, with only electrical wires entering the envelope that contains freon.  However, they use a lot of aluminum, as it is a superb themal conductor, and cheap.  Accumulated dust in the cooling coils and fins tends to hold water, a perfect storm for aluminum corrosion.  Good chance you have a pinhole in the condenser.

Likely you have a foreign made pile of junko, so there is nothing in the unit designed to help you. Every inch of tubing in the unit is welded, and there are no spare parts to be had for the mechanical package.  (Control board, yes.) 

If you have an old school solid American unit, it most likely takes the long-discontinued R12 or R22 Freon, so you are paying top dollar for old stocks. 

Refilling is dangerous

Contrary to what comic books say, a Freon accident will not turn you into Mr. Freeze.  Cold is not like fire, you don't instantly feel the pain, you can expose yourself long enough to do damage before you realize it.  

And there you are, using third party parts of dubious quality, since no manufacturer endorses even authorized repair of the Freon containing machinery - it is not economical even if they made it easy, which is why they don't.

Who pays the electricity bill?

If it's you, you're paying again to run an old inefficient unit.  Ten years ago I bought a solid Sears unit that did 5600 BTU and drew 5 amps.   My friend just bought a private label no-brand junko unit that did 8000 BTU for 5 amps.  His unit runs 1/3 less often, so he's saving about 200W or 1 KWH every 5 hours.  An older unit would be even worse. This is due to Federal laws requiring ever-increasing efficiency. Do the math... Then imagine what a quality unit would do. 

Even if you can get 5 more years out of your old unit, the new unit will pay for itself in electricity costs alone, to say nothing of all that other stuff.  

So fixing an old unit is a false economy.