Switching over iPhones/resetting old iPhone

Im in the scary process of resetting my old phone. I don't doubt this is an easy process but leave it to me to have an issue. Ive been using my new phone for a while now (everything has carried over; icloud, pictures, notes, music are all backed up). The last time I did this and transferred everything to my new phone, I reset my old phone only to watch everything from my old as well as new phone delete. Im paranoid this is going to happen again. The last time I did this I was signed out of icloud but lost everything. I currently still have everything on my old phone as it was last used, just no SIM card.

Thank you in advance!