start with a few tiles (can be made inside construct) so you know the type of tiles it represent, build a simple structure that shows all you gonna use, like a test room: solid floor, jump floor, doors, chests, walls, decoration, object...
then you save as a project file so it will maintain the tiles .png now you can open the Photoshop(i use SAI because is lighter for my pc...) and pimp the tiles while you replace then inside the root arquive from the construct 2 project folder( and keep a backup project).
this way you dont need to rebuild the test room, while you still can see how the tiles work together.
after this i think the best way to build levels is using a tile editor like, DAME and build using the tiles you made and then export the map to construct
here is some 16x16 pixel art
and keep simple at the beginning, so you dont give up
hope it help