This guide is for patch 1.03. Currently all Starting as achievements are bugged and you need to complete it within one playthough, so no quitting to the menu.
Make sure to set your settings to Ironman Mode - you will know if acheivements can be unlocked as there will be a gold trophy.
You want to select Matilda di Canossa in the 1066 start which is under Rags to Riches, its considered a medium difficulty start.
The aim is to create the Kingdom of Italy, have 50 Dynasty Members alive and form a Holy Order. Make sure the last thing you do is form a Holy Order otherwise you might not get the acheivement.
Some tips for getting the acheivement easily:
Matilda starts with a bonus to the Diplomacy lifestyle, focus your perks down the Family Heirarchy tree, perks like Befriend and Friendly Counsel will help.
Get a husband with good skills and potentially a fertile trait. Use your Court Chaplain to fabricate claims on some of the small countys that can be taken easily. The easiest way to create the Kingdom of Italy is to get a claim on the Duchy of Lombardy. Befriend the Pope and you can Request Claim - this isn't always possible as the Pope doesn't want to give the claim to a Woman but usually works; otherwise use your spy master to get a hook.
Get 15 counties within the De Jure area then have 500 gold to create the Kingdom of Italy. Once you have the Kingdom of Italy its just a waiting game - tick matrilineal when you marry your daughters so there children are of your Dynasty - eventually you will have 50 alive at the same time. Make sure you have 500 gold, 1000 piety , level 3 devotion and 60 relationship with the pope and then you can create a Holy Order and the achievement is yours.
The hardest bit is getting the Kingdom of Italy - if you cant get the Duchy of Lombardy, use the Meritocracy perk down the Stewardship lifestyle and get a claim on the Holy Roman Empire and Kidnap the Emperor to get the Empire then create the Kingdom.