[Q] How to install rom when all memory is internal? topic

I have an ls970. It's rooted and has twrp 6.3.3 installed by freegee on it. Everything is working very well. I'm just curious how to install a rom on it when your supposed to erase all data on the device. I have already backed up my entire os and saved my efs to my pc. i backed up twrp (after rom backup of course), freegee, and efs folders to my pc as requested. If I erase everything on my device, my rom won't be there to install. I've never installed on a phone that didn't have external storage (which is where i normally put my rom). How do you guys manage this? thanks in advance for the help!