Hello @chelseavan_der_ark! 
Welcome to the community! You may need to change your printer’s settings to Fit to Page and make sure you choose borderless printing, as well. You can change the printer settings when you choose the print from printer dialogue or something along those lines. You may need to select Advanced Settings or Preferences to find those settings. Also, I recommend practicing on regular copy pape, preferably on a light grip mat, as the copy paper will tear when you try to remove it from the mat — until you’ve made sure everything is just right.
Also, Print Then Cut can be very tricky, so I’d like to direct you to a video made that includes the way I like to do a PTC. It’s the only way I’ve found that works. From IdeaStudio to Finished Envelope — My More Self‑Love Project Video!
I also explain my PTC process in this post: No lee las marcas de registro [RESOLVED].
Let me know if this works for you, as it may help others who come across this post.
If you have any questions, don’t hesitate to reach out. I’m here if you need me.