To do a double crochet decrease depends on which stitch you are using. When you are working with a double crochet, to decrease at the beginning of a row, make the turning chain but then miss it and insert the hook into the following stitch.
At the end of the row, in order to make the edges match, miss the last stitch and work into the top of the turning chain.
If you miss the turning chain, you will get a stepped edge, which is not as easy to join. The same method applies to the basic stitches, but the edge will be neater if you also make one fewer turning chain than usual at the beginning of the rows.
If you are working a shell pattern, work half a shell instead of a full one at the beginning and end of a row. Where threes already only a half shell at each end, slip stitch across the one at the beginning of the row and miss it completely at the end.