Instead of the vertices filling on the correct side, they meet across the z axis. Thank you 🙏 (I hope that you guys can see the picture)
I’m sorry, am I doing something wrong? I tried to use the options that you’ve told me to. I want it to look somewhat like this:
But instead it now looks like this (please help): 
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$\begingroup$ Hi. For future reference: take-a-screenshot.org $\endgroup$ – Ray Mairlot Dec 31 '18 at 19:03
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You've enabled the Adjust Edit Cage To Modifier Result option of the modifier (fourth button), so you're confusing the real mesh and its mirrored image, and you've created the new vertices in the mirrored space. Deactivate this button and it will appear clearly. Also, activate the Clipping option so that the vertices stick to the axes and don't trespass.
