Section
Start with one section at a time rather than trying to complete the entire puzzle at once. This will allow you to focus on the details and shapes needed for each section before moving onto another. For example, if you are completing a 3D puzzle of a globe, start with a continent and complete that before you move on to the next one.
Identify
Identify distinctive features and edges and shapes and start with those, as they will be easier to fit into place as opposed to a piece that has no clear identification with any part of the puzzle. Remove the easiest parts of the puzzle first, assemble them and put them to the side until you are left only with puzzle bits that are tricky.
Assemble
Assemble the flat panels of the puzzle pieces you've managed to put together and connect them, creating the 3D shape that they are supposed to take on. Pieces will be flat or have curved sides, which will help you assemble the shape of the entire puzzle.