They can be made anywhere in the tissue, either at the beginning, middle or end. The easiest way is to work to increase two stitches on a stitch basis.
In the place where you have to increase, work stitch normally and then insert the needle in the same stitch to thereby form the increase.
If increases are made in the center of the tissue it is advisable to mark the spot with a thread of contrasting color, so they can be easily located in the event of having to make further gains at the same height in the following rows. To increase 2 stitches instead of 1, proceed in the same way but crocheting 3 times at the same point.
To increase number of stitches at the start, for example 4 stitches, you must weave a number of chains equal to the number of stitches to be, in addition to the necessary stitches to continue crocheting normally.