Depending on what air conditioner you have and what manufacturer it is, there are different rules and regulations when it comes to installation. It is always recommended to have an HVAC contractor help with the installation as most of the manufacturers need a certified air conditioner installer to sign off on the installation certificate. If you do not have such signature, the warranty of the air conditioner gets voided and you get left with a system with zero warranty. You can always do all the handy man related work yourself and get an HVAC contractor to just help with the few important installation procedures. This can save money and time on both sides.