The easiest decrease is the "k2tog" or "knit 2 together." You simply knit as you usually would, but you knit through two stitches instead of one. Just set your needle tip one extra stitch back from the first one, and scoop together those first two stitches on your left needle. Easy peasy!
Once you have mastered this basic decrease, you may start noticing the way it looks. There are two basic categories of decrease: left-leaning and right-leaning.
This can be a somewhat difficult concept to grasp, because its effects aren't immediately apparent. It isn't like a cable, which visibly slants to the left or right. In large part, you have to just sort of take it on faith that (for instance) an SSK slants to the left, while a K2TOG slants to the right.
The left-leaning and right-leaning decreases are important in the way they interact with a left-leaning or right-leaning line of decreases. Imagine you have laid a raglan sweater down on the table in front of you. The line of raglan decreases on your left (the sweater's right) side lean to the right, like /. The line of raglan decreases on your right (the sweater's left) lean to the left, like \.
If you choose a decrease stitch which leans in the opposite direction of the line of decreases, the stitches will seem to disappear. Whereas if you match the directions, you will end up with a line of stitches which seems to march along the decrease line at an angle.
The easiest way to get a feel for this difference is (wait for it…) to make some swatches. Play with the placement of the decreases, the direction of the line, and which stitches you use to decrease with. For this swatch we will create a line of decreases which slants left. The first half of the swatch will use a left-slanting (i.e. matching) decrease. The second half of the swatch will use a right-slanting (i.e. opposite) decrease.
Left-Leaning Decrease Swatch
* A ball of waste yarn at least golf-ball sized. I like to use a DK-weight yarn for something like this. I always have plenty of leftover balls of yarn like Cascade 220 and Patons Classic Wool. Choose a light-colored yarn if possible, since it will make it easier to see what the stitches are doing.
* The appropriate needle size to knit that yarn.
* A stitch marker.
1. CO 20 stitches, in whichever yarn and needles seem appropriate.
2. Knit, KFB, place marker, SSK, knit to end of row.
3. Purl back across the row.
Continue until your decrease line is at the middle of the row, with 10 stitches on either side of the stitch marker.
4. Knit, KFB, place marker, k2tog, knit to end of row.
5. Purl back across the row.
Continue until your stitch marker moves to the end of the row.