Ok so looking at your stem, there's a little number on the come...it will say like +4 or -4. If it says +4, you can switch it to -4.
Essentially the spacer is always inserted from the bottom of the stem. So don't insert it from the top.
What you want to do is remove the stem bolt, loosen your stem, pull your stem off the steerer tube, pull out the spacer and spin it 180 degrees, reinsert it, put the stem back on, ensure the big bolt in the headset it tight (this is the expanded plug), semi-center your bars, put your stem cap on, tighten it to the proper preload, center your bars, tighten the stem.
That's it. Now realize that when you do this, your bars will be rotated down in the stem now, so you may want to loosen the 4 bolts there and rotate them back up (depends on comfort).
How that spacer works will be obviously you when you take it out. It's beveled/tapered/thinner on one end than the other. So when you spin it, the stem will face down instead of up by the degrees listed on the side there (on my bike it's + / - 4).
PS - I am on my phone so all the typos, I'm not going to go back through and change.