I assume you have a nexus5 from your Sig, if so take these steps
Copy new ROM + gapps to SD
Reboot to recovery
Do a nandroid backup
Do a factory reset (as in wipe all but internal SD - so no extra data wipe)
Reboot - profit!
If you want to wipe all, which can be nice
You will need to use adb sideload to install ROM, this isn't hard and what I do most often as I'm ocd when it comes to a clean wipe, just be sure to backup all data from SD to PC first!
Sent from my Nexus 4 using XDA Premium 4 mobile app