Make sure that the pieces you're going to attach to each other are completely dry, otherwise you are increasing the risk to break them.
If there are edges that need refining, do it before attaching your pieces.
When your pieces are ready, apply water with a wet brush on the parts you're going to join together. Using your syringe, extrude some clay on one of the pieces on the wet part, and then attach the other. Very carefully with a spatula, or your needle tool, or a clay shaper remove the excess clay from the seam; smooth the seam with a wet brush. Don't overwet the piece! It could break. Dry the piece and check it again. If you're not happy, add some more syringe and smooth the seam.
Don't leave any gaps between the attached pieces. If you want to attach more than two pieces you must repeat the same process. Don't forget to dry your piece after every step you have added water on it.
In case you don't have a syringe you can use paste; it works very well too.
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