A/C is not rocket science, however it does require special tools and gauges to do it right, all of which your average home mechanic does not have.
For a modern R-134a fitted car I would not be adding Redtek to "top up" the charge. Take it to a shop and have them leak test it or do it yourself to check your system. Then have it evacuated and filled with the proper amount of 134a once you know everything works fine.
However if you have an older R-12 system then by all means use the Redtek stuff after evacuating the old R-12 (at a shop), and leak and vacuum testing it. Or better yet buy some R-12 on eBay (that's right you can still buy it there)
89 535iA ; 93 M5