This Achievement is 60 percent luck-based.
If Haesteinn dies at all YOU MUST RESET!
The easiest kingdom to invade is Brittany, just north of you.
However, in every run I had, the Bretons will ally with West Francia every time.
Fortunately, there are 2 ways around this
1. Kill the woman in the marriage for the alliance, I never failed to kill her since you get a lot of money to start the campaign to bribe agents but it is also RNG based since 95% in CK3 means 60%.
OR
2. Rush down the Breton capital and battle their forces to Capture the king before West Francia claps your cheeks. This is even more RNG heavy.
GREAT JOB! You now have all the titles to make the Kingdom of Brittany!
Uh-oh, Pagans need 2 duchy titles to make the kingdom and unlike CK2, Brittany is a single duchy by itself.
IF YOU HAVE LESS THAN 4 BRITTANY PROVINCES RESET! I had to surrender to foreign powers (West Francia & Aquitane) in order to kill the other duchy but as long as you have enough provinces it'll be good enough. RNG
There are 2 RNG based duchies to rush down ASAP
1. Cornwall, they are weak depending on their alliance, I lost a run to a West Francian backed Cornwall who outlasted Haesteinn, however, you will have enough Prestige to take the duchy and your spawned army can still carry you to victory
OR
2. Leinster, the Irish are in shambles and hopefully, the Vikings haven't taken too much so you could build a duchy over there, Sudeyar screwed me in a second run when they raided me to oblivion and took my land.
GREAT JOB! You have conquered the land!
Uh-oh, you seem to be low on money To make the Titles!
It's time to raid people! Your spawned army will destroy everyone in England, Wales, Scotland, Ireland, and even Navarra if you run out of people to rob.
You need to create the 2nd duchy and the Kingdom title, which requires a couple hundred gold each, so get looting and pray for the event that gets you more money!
I lost a run when Haesteinn died when I needed just 1 more gold to get the achievement.
TLDR
1. Brittany kill
2. Cornwall or Ireland Duchy conquest
3. Raid for money for titles
4. Pray to RNG gods for long healthy Haesteinn