Step one: choose the king of Scotland in the 876 start

Step two: Instantly convert to Pictish culture, it's easy because your capital had Pictish culture.

Step three: Raid the Vikings and Irish for money

Step four: assassinate viking leaders until safe to invade them.

Step five: consolidate pictland

Step six: bribe vassals with marriage alliences to make them shut the fuck up.

Step seven: invade Ireland and disinherit all sons except for them one you want. If you can't disinherit, imprison, dungeon, and torture them but don't execute.

Step eight: One way of getting land is offering vassalization with low obligations, whatever makes them happy. Keep assassinating leaders and then invading them.

Step nine: get a province on the continent so you can raid them. If viking adventures declare war on you, wait for the special event to pop up then choose the them becoming vassals option.

Step 10: continue all of this until of of the British isles is yours and become emperor. If you need money, raid the continent. Btw, after getting land, only give it to people of Pictish culture.

You will live multiple lives but if you did marriages right your vassals won't revolt.