This is a very difficult process to pull off manually now that the Glowforge Passthrough software is being Beta tested. I believe it has been re-opened for sign-up, and while it might take a while for them to get you access to it, it will be worth it. The Glowforge software does all of this for you so you don’t have to be so careful.
The sign up for the Beta testing is here…if you do sign up for it, make sure you have run the Calibration program first to calibrate your machine, and read the instructions for the Beta testing.
If you don’t choose to sign up to test the Passthrough software, or if they already have enough testers for it, all I can tell you is to be extremely careful with how you are cutting the sections and placing the indexing marks. Precision is key.
Oh…and if you are working on some SVG file that you purchased or downloaded that might have been created in some other program besides Illustrator, it WILL introduce sizing errors into the design. Each program has it’s own default export size settings, and they are not the same in Inkscape or Affinity Designer or Illustrator. That makes sizing errors with SVG file types very common. When in doubt, always copy the file contents into a new 20" x 12" file and work from that. Check that the size is accurate inside the design program of choice, then when you save the SVG file for use in the Glowforge it will come across as correctly sized.
That’s usually what causes size issues and it happens before you get the file into the Glowforge interface. 