I always start with the "process of elimination".
visually inspect the rear derailleur...is the jockey/idle pulley cage straight and in proper alignment?
check chain stretch with a "check tool"
check cassette with a "check tool"
check rear derailleur hanger with alignment tool
check rear derailleur limit screws...if hanger is out of alignment you should have to reset the limit screws
check cable tension to assure correct derailleur alignment after all of the above have been checked and/or set
a skipping gear can be the result of a few things, usually a stretched chain not not always.