Just ensure it's completely clean...
When I apply them to commercial vehicles I use a silicone remover to ensure long term adhesion but this makes marks more likely when finally removed. (Typically I used IPA... no, not the beer
)
On friends vans I've applied over existing finishes and with a good vinyl (Avery etc) I've had no problems with 6 years and over. (My van has them for over 5 years... and this is on top of wax) Cheap vinyl is another matter.
Use an application fluid when applying to ensure you get no bubbles etc. Apply it liberally and squeegee out. (A mix of water and a drop (literally a drop or two) of detergent can be used. Not quite as effective but works well)
Personally I apply, squeegee, leave for a few minutes then apply fluid over the application tape (makes the application tape easier to remove and prevents it lifting the graphic, remove the tape carefully then squeegee out any bubbles which if you use an application fluid will be liquid filled and not air and as such easier to remove.
Lots of different ways of doing this but this works for me.
It's a bit of a hobby for me... up until recently I used to make the club stickers etc for this forum when the proceeds used to go towards charity.
Hope this helps.
Rgds
Mark