To increase one stitch, insert hook in the back loop (the back ridge of single crochet) of the next stitch and pull a loop through.
Continue casting on loops in the normal manner to the next to last stitch.
Pull a loop through the next vertical loop, pull a loop through the back ridge o the same stitch,
then finish the row in the normal manner.
Work the cast off row in the normal manner.

The arrows indicate the added stitch at the beginning and end of the row.

This image shows the increases after four rows.

To decrease one stitch, insert hook under the next 2 vertical loops and pull up a loop.
Continue casting on loops in the normal manner until 3 loops remain.
Insert hook under the next 2 loops and pull up a loop, then finish the row in the normal manner.
Work the cast off row in the normal manner.

This image shows the 2 decreases.

This image shows the decreases after 4 rows.

To increase several stitches at the beginning of a row, (the right edge of the fabric), work a return row and then chain the required number of chains. (I refer to this as "chain back").
Work a Forward Row into the extra chains and across the fabric.
Pull a loop through just the front loop of the last stitch because you will be adding more stitches.

Add extra loops to the left side of the Forward Row.
Pick up a loop of supply yarn between thumb and forefinger and give the loop 3 twists in a clockwise direction.
Place loop on hook and snug it gently to the edge of the fabric.
Add as many loops as you need in this manner.

Alternate Left side Extension:
(a bit complex but has a very nice bottom edge)
Take a second piece of yarn (I am using beige only so it will noticeable),
Make a chain the needed length and fasten off.
Work from the back side of the fabric and fasten the added chain using the starting tail
to the last stitch of the row to be increased.

When done correctly, the back ridges of the chains will be to the front of the fabric.

Continue with the forward row, pulling up loops through the top loop of each chain.

This Alternate Extension sample is done in Knit stitch to show that the first row
of each extension must be done in Simple Stitch. All other rows will be in whatever stitch you want.

Work the usual Return Row across the new loops and the fabric.
Image shows increased fabric after 4 rows.

To decrease several stitches at one time at the beginning of the Forward Row,
just Bind Off the required number of stitches,
and continue with a Forward Row. To decrease several stitches at the end of the Forward Row,
leave the unneeded stitches unworked.
Image shows decreased fabric after 4 rows.
The unworked stitches on the left side can be finished later by working a Bind Off with a second piece of yarn.
Image shows fabric after 4 rows.
Single arrow points to the Bind Off side; Double arrow points to unworked stitches.