Mmmmmmm 2 things:- training and experience my friend.
Myself I haven't found a GPS RNAV approach in Australia too difficult to do without VNAV ( Boeing ) or FINAL ( Airbus )
There usually aren't that many Altitudes to meet anyway.......
Can't your Jet fly the track on AP while you setup the required ROD and then monitor it??
Basic stuff Dick......don't you remember doing NDB's in a twin, hand flown with an engine out AND with a 30 kt tailwind and turbulence on the final segment?????? I do and these modern jets are a breeze in comparison...
Where have all the skills gone??