It say you can colonize a county in the wilderness but i don't see where you can do that

I've been playing the HECK out of an Aragorn run trying to rebuild Arnor so I've become fairly familiar with the whole PROCESS. And it IS a Whole Process.
It might work differently for Dwarves, but for the Races of Men it works something like this:

Find a Wilderness county you want to settle, select the barony that acts as its county capital (it's usually the one you get if you click the county) that's adjacent to your realm (I know you can colonize Andulanoli off the coast of Lindon, but I'm not sure how, apologies)
If the selected area says the name of the barony and that it has no holdings, the same "card" on should have a way to select between baronies in that county (below its De Jure hierarchy), the one on the left with a crown symbol is what you're looking for.

Either way, once you've selected the county capital barony there should be a button in the center that says "Colonise [County]" click that and you get a Settlement/Wilderness Holding there.

To upgrade it into a standard castle, town, or temple, you have to remove the "blocker" buildings it'll have (it'll start with 2, rarely an event will give it a new one) by upgrading it.
This seems counter-intuitive but you WANT to upgrade the Problem Building.
You'll also need to upgrade the "main" structure (settlement) to 3, which will take a couple years, and get the development level in the county to 3. Once that's done it's like feudalizing a Tribal holding into a Castle holding, there's little buttons for it.

I PERSONALLY have found the most efficient means of claiming wilderness to be thus:
Claim the county, remove the blockers and build the Construction Guilds building, which increases construction and development speed (there are others "colony buildings" to increase levies or taxes but it'll slow down actual development)
Once you've got Construction Guilds, upgrade the holding twice.
Either have your Steward developing the holding from the start or, if you've got several going at once, whichever one's closest to completing the process. Getting those 3 dev points is the most lengthy part of the process... it can take a VERY long time even WITH a great Steward on the job.

Once the process is complete you'll have a brand spanking new Castle, City, or Temple to upgrade as normal!

Thanks for the info. Yeah, I stumbled across the way to do it myself recently, finally. But this gave a few more pieces I wasn't completely sure about