My two cents ...
The painted surface needs to be smooth, dry, no cracks, bubbles, drips, dust etc. deal with all the normal painted surface prep. Surface preparation is always important, not to be rushed. The devil is in the details.

Also rough it up a little with 60-80 grit so that the contact cement has a little more to grab onto.
Pay attention to what the paper backed and contact cement folks tell you about how to get a good job from that. You can pretty much ignore that its painted other than it will effect the prep a little.
The main thing is you don't want to use a glue that assumes its going to soak in, it won't, the paint soaked in already. Nothing much in the way of pores is left. That is why contact cement and paper backed veneer.
Brian