Decrease in Half Double Crochet (or how to hdc2tog)

All of your taller basic stitches will essentially be the same thing; you'll begin the first stitch, leave off before finishing the final step of it, crochet the next stitch and then finish the two together. Here are the instructions for half double crochet decrease:

1. Yarn over hook.
2. Insert hook into stitch.
3. Yarn over hook and pull through. There should be three loops on your hook.
4. Insert hook into the next stitch.
5. Yarn over and pull through. There should be four loops on your hook.
6. Yarn over and pull through all four loops.

Decrease in Double Crochet (or how to dc2tog)

We will go through one more stitch so that you can be sure that you have the hang of decreasing. Let's do the double crochet stitch (or double crochet two together):

1. Yarn over hook.
2. Insert hook into next stitch.
3. Yarn over.
4. Pull the yarn through the first two stitches. There should now be two loops on your hook. Notice that this is a normal double crochet stitch up to this point. There is only one step left but you're going to wait because you will be finishing this double crochet with the adjacent one to make a single stitch across two from the previous row.
5. So, leave those two loops on the hook, yarn over and insert your hook into the next stitch.
6. Yarn over and pull through the first two stitches. There should be three loops on the hook.
7. Yarn over and pull through all three stitches.

Cluster Stitches

Note that working multiple stitches across a single stitch like this is also called a "cluster". The 2dctog described above is, for example, a "two double crochet cluster stitch".

You could also work more than two stitches to create a bigger cluster. For example, a cluster of four double crochet stitches would be worked by crocheting 4dctog, where you do the same thing as you do with 2dctog, except that you leave the first three stitches unfinished (instead of just the first one) and finish them all together at the same time as the fourth one. So, the steps for the four double crochet cluster stitch would be:

1. Yarn over.
2. Insert hook into next stitch.
3. Yarn over.
4. Draw yarn through stitch.
5. Yarn over.
6. Draw through 2 loops on hook.
7. Repeat steps 1-6 three more times. You will now have four unfinished double crochet stitches next to one another. There will be five loops on your hook.
8. Yarn over and draw through all five loops to finish.

How Many Times to Decrease

Now that you understand the basics of decreasing stitches, let's discuss how many times you will do this per round or row. The answer is ... let the pattern tell you! If it says to decrease once, then you just do this one time (crochet two stitches together into one) and proceed with the pattern as normal. If, instead, it tells you to repeat the decrease across the entire row, you would turn each of the pairs of two stitches into a single stitch by repeating the "crochet two together" over and over.

Summary on How to Decrease in Crochet

To recap, anytime that you are going to decrease in basic crochet stitches, you will be working two stitches together as one. You will begin the stitch and work it as normal until the final step of that stitch. Leaving that final step unfinished, you will then work the next stitch as normal. When it comes time to complete the final step of this second stitch, you will pull the yarn through all of the loops on the hook, completing the final step of both stitches at the same time, effectively turning the two stitches that are side by side into one stitch because they now share a final shared stitch at the top.

Alternatives to Decreasing in Crochet

The method described here is the one that is most commonly used to decrease crochet stitches. However, there are other methods that different crochet designers may teach you in their patterns. For example, Stitch Diva has some great instructions for doing a double crochet decrease where the stitches are joined at the bottom of the stitch instead of at the top! You can come back to this method as a default but don't be afraid to try new things and see what you like best!