very important to use proper procedure.
hook up the pressure gauge to the system at the outer unit and connect a vaccum pump. completely vaccum the system and then check reading on pressure gauge. if it remains constant after about 30 minutes then means your system is ok with no leaks.
hook up the referigerant cylinder, check if your ac takes r22 or r410. new inverter ac that i know off take r410. gas can be of dupont, honeywell. usa or spain made.
put the cylinder on a weight machine. note the weight. gradually charge the system and note the weight decrease of the cylinder. most 1 ton ac systems use about 900gram of referigerant. 1.5 tons use about 1.4kg gas. this can increase or decrease depending on the length of piping your are using.
charging by weight is the most accurate method of gas charging. start up the ac. check compressor amperage with clamp meter. should be same to the rating given on the ac outer unit.
check system high and low pressures, they are mentioned in most cases on the ac outer unit. quantity of gas charge is also mentioned in most cases.
In case the pressure does not remain constant after vacuuming the system that would mean you have a leak. in that case. use the compressor to pump the system with air. (nitrogen is a better option) close all valves and compressor. and then do a puncture check with soapy water along the lines where the gas flows. repair leaks with magic depoxy( temporary solution) or with proper oxy acetylene gas brazing.
then follow method of gas charging.