You can increase double-crochet stitches by adding a stitch at the beginning of the double-crocheted row. Increasing double crochet with this method creates a smooth, tapered edge to your piece.
Try your hand at adding 1 double crochet stitch at the beginning of a row:
Make a swatch of double crochet stitches.
Chain 3 (ch 3).
This is the same number of chains required for a double crochet turning chain.
Work your next stitch into the first stitch of the row.
Finish the rest of the row as you normally do, working 1 double crochet stitch in each stitch across the row.
Take a look at the symbol used in pictorial crochet diagrams to represent an increase at the beginning of a row: