You can use your fingers to pinch the stems back, or you can use a sharp scissors. I just take the stem between my forefinger and thumb, and press my fingernails together to cut through the plant stem. If it's easier for you to use a scissors, or even a sharp knife, that's fine, too.
You want to cut through the stem just above a node. The node is the kind of swollen part of the stem, where the leaf emerges. I think I would cut about 1/4 to 1/3 of an inch above
the node where you want your new stem to start to grow. You can remove the leaves from the bottom part of the cutting you removed, and just stick them into the pot with the rest of your plant. (Use a pencil to poke a hole in the soil.) The cuttings will grow new roots and create even more stems for you, adding to the fullness of the plant.
If you prefer, you can use the cuttings to start new plants. Just root them in water, or stick them in a small pot of moist potting mix, sand, or vermiculite. (I just take a little pot of potting mix, use a pencil to poke a hole, and stick the cutting in. Put several cuttings in one pot. Water it well to settle the potting mix, and place it in a bright spot, but away from direct sunlight, until new roots form.)
Does that help?