The galaxy nexus doesn't have room for an sd-card, so either you are following a guide for a different phone, or you have misunderstood something.
If you still have a working recovery, then one way to do it is to adb push the rom and gapps files from your computer to your phone, and then flash them with the appropriate wipes.
Another way is to use an otg-cable and a usb storage.
Whether or not you can upgrade (and in most cases, the issue IS storage space), the best way to start is as close to "virgin" as possible, as the GNex physically is incapable of taking an SD card (by that, I mean it lacks a slot for one; however, it has more physical storage than my Lenovo A7-30F - which makes up for it by taking microSD cards up to 32GB in size). If you have a working Google account (remember, EVERYTHING Android starts there) do a factory reset from the phone (yes - it WILL kill all the customization that has been done with the phone - including blanking the data on the phone; that is, after all, the purpose OF such a reset).
Here is the process that I used for my own GNex, condensed - when I got it, it was going to be leaving VZW for a neutral MVNO (Tracfone) which meant that I didn't have to keep anything VZW-related on the phone; hence a factory reset would certainly do. I already had a Google account that had nothing to do with VZW on hand, as well as the Skipsoft Universal Android Toolkit (which has specific settings for the Galaxy Nexus as far as rooting and installing initial recoveries; I also have both Odin and Samsung's own Kies as backups) - however, Skipsoft did the trick with both root and installation of a third-party recovery (TWRP, which is my preference). Then came the hard part - choosing a community ROM; LOTS of choice there! My first choice was the safe-choice - CM13 (which had recently gone official)), so I flashed the then-latest nightly and GApps 6.0 pico (heeding the warning specifically regarding the GNex and GApps nano being too big). I had ONE problematical app - however, I could work around it (as it is a common app with my tablet); but then I ran into a newer community ROM based on (egad) Nougat - in fact, it's based on 7.1.. I was able to use the same recovery (TWRP 220.127.116.11) I had, but like an idjit, I forgot my own reminder about GApps bigger than pico and tried to flash OpenGApps 7.1 nano - which promptly flubbed-up. Result - NO GApps and no root, either. Factory-reset time; which meant I needed to re-root (Skipsoft Toolkit) and reinstall TWRP (Skipsoft Toolkit there, too). Rather amusingly, I STILL had 7.1 installed - I simply needed the proper GApps - 7.1 pico (vice nano). Fixing my earlier PEBKAC error, I transferred the proper GApps and flashed them using TWRP, and I was right as rain. The only thing I'm awaiting is the SIM Kit from Tracfone (ordered and enroute via USPS).
Current ROM - Unlegacy Android 7.1_r1
Current GApps - 7.1 pico
Current Launcher - Microsoft Arrow
Current Assistant - Cortana
Enroute - SIM Kit for Existing Service Plans (Tracfone - migrating number from lifeline feature phone; this is, in fact, permissible)
Enroute - Cruzer Hardcase via Amazon (ordered today)