Summing up from the comments:
This very much depends on...
- the ROM used
- the update format chosen for a specific update
Some ROMs (e.g. CyanogenMod) come with their own "updaters", where special rules might apply. Specifically, they might deliver "patch updates", where only some files will be replaced. In those cases, no further action is required (except for, maybe, a reboot).
Some other ROMs ship in specific formats, and might require special tools to be flashed (e.g. odin or heimdall for Samsung devices).
Again other ROMs are delivered using the
update.zip principle, where a custom recovery is needed to install them. This
update.zip may be a complete ROM, or also just a patch. Instructions usually are to be found where the updates are offered for download.
In all above cases, some "initial steps" required for the first ROM install can be skipped and must not be repeated, including...
- rooting your device (custom ROMs usually come pre-rooted)
- unlocking the bootloader (unless you've re-locked it)
- installing a custom recovery (when it's already there, no re-install is required; though there might be updates to this as well, and a newer custom ROM might require a minimum version here as well. Again, instructions should say so)
It's always a good idea to perform a backup before installing updates (or tampering with system stuff in any other way). Points to check for include:
- nandroid backup: a full "sector-wise" copy of all the important partitions. If things go wrong altogether, this enables you to revert to the exact state of when these backups were created
- logical full-backup: this is performed by tools like titanium-backup, and usually includes all your apps and data. So if something is missing after the ROM update, you can substitute the necessary parts. (note that Titanium Backup also can do that with Nandroid-Backups)
- adb backup: quite similar to the previous one (see the backup wiki for details and differences).
Note also that an ota-update is quite unlikely to happen with custom ROMs. Some even include an app named
fota-kill to "silence" the related update check. They either bring their own updater, or none altogether. In the latter case, the user has to check himself whether and when updates are available; often a "reminder" can be setup by subscribing to the corresponding thread on XDA (or wherever the ROM comes from), but actions have to be taken manually (as described above).
As I've shown until here, videos on updating your ROM most likely do not exist as the process already has been described as part of the initial video; though a few words like "for updating, skip to index ab:cd" would be a good idea (and could be suggested in the comments).
A special case are the google-apps (short: GApps). Whether you have to re-install them or not mainly depends on whether it was a "full update" (complete ROM), or the update just replaced some files. But it's easy to figure: When finished, just check if they are there and working. If not, you obviously have to flash them again. As a full-update replaces the entire partition content, GApps would be gone with that; a "patch-update" would just replace/add some files, and not touch the GApps, so they'd still be there. GApps are never part of a custom ROM (if they are, the "ROM cook" is in trouble due to license issues).