Using CM with the magik.net plug in and then enabling skin editing mode will make things much easier, because its showroom will update the skin in real time each time each time you save your .psd (no need to export).
As I don't do proper 3D painting, one trick is to first place a part that you want as a reference for alignment, like the door face, then make a backup of the layer with the motif. Then you have one choice to make, a) if you want something to look like a real sticker that was trimmed on both halves after being glued over the car b) or something that will align perfectly and also visually continues inside the gap.
For a) You can cut the door sticker via moving a selection you made that cuts it perfectly at the wireframe limit on the exterior of the panel without going trough the gap (or overlapping just a bit as I prefer), so you'll move it to the place it'll continue visually. To make it more seamless, you should trim that second part a bit to make it flush with the gap (or again, overlapping it just a bit) between the halves (you can invert the selection and then offset, delete). -> This is looks good for stuff like pub stickers or roundels. (On my skins for the Veloce GTS you can check the Bacardi bat roundels cut this way, where a part of them is missing that corresponds to the gap space)
For b) You when cut around the door, you can let it extend through the gap. Then in the second part, instead of using the remaining part, you re-cut the original motif with another border going through the gap (that will overlap a little with the previous cut) so that it seems like the design will continue and exist inside the gap instead of letting the car base paint show. Most times, there's no reason to do this, but a complex vinyl wrap might look more seamless on wide gaps or a contrasting base paint.
