The safest way to do this, without using the proper equipment, would be to vapor charge. The can will get cold and slow the process, but this can overcome be submerging the can in a bucket of warm (not hot) water. It’s easy to “slug” the compressor with a liquid on the low side. Your condenser fan should come on during charging. If you have a thermometer to monitor the center vent outlet, charge to the point the outlet temp stops dropping and starts to rise.

The best way to charge 134a systems is by weight and outlet temp.

Edit
After seeing stevebfl’s post, I'm sure it's in reference to the bucket of warm water. A point well taken. Unless you are familiar with the dynamics and pressure temperature relationship of refrigerants, (as the questioner is not) this is not a good idea. You don’t need to use the water at all or you could use water at ambient (or cooler) temperatures and this will work well too. The reason to use the water is only to lessen the amount of time to charge, giving the canned refrigerant a source of thermal energy.

Also be absolutely sure you are connected to the low side, usually the larger diameter fitting on R12 systems and smaller on r134a (if properly retro-fitted). The low side is the line from the evaporator to the compressor.

Steve’s concern for safety is admirable and I should not have mentioned the warm water. Refrigerant can be very dangerous: one drop of liquid refrigerant vaporizing on your eye could cause blindness. Always use safety goggles.

If you wish to work on your own A/C system you should at least know the basics. It would be a good idea to learn by reading or attending a class on air conditioning. We don’t want to save money at the expense of our health.