Character creation rules follow my general house rules, with the following exceptions:
Your character creation options are the following:
Your rolled array, 17, 9, 11, 18, 13, 11
My array, 18, 16, 14, 12, 10, 8
32 point buy.
You start at level 5.
You start with no items.
You can use the optional multi-subclass rules as follows:
When you level up, you can multi-sub-class. When you do this, treat your second subclass as a separate main class. If you have caster levels, combine them as normal. Each level you take of the multi-sub-class gives you one level of subclass features. You may not get subclass features from levels above your primary level in the class.
Example
Consider a 15th level Thief Rogue that wants to put levels into mastermind as well.
At 16th level, they decide to multi-sub-class into Thief Rogue. They do not get the 16th level ability score improvement. Instead, they get the first level worth of subclass features from Mastermind, the third level features Master of Intrigue and Master of Tactics. They get an additional hit dice.
At 17th level, they do the same. They get the second level worth of subclass features, the ninth level feature Insightful Manipulator. Their proficiency bonus goes up, they get an additional hit dice and an additional sneak attack dice.
They continue at 18th level, gaining Misdirection and another hit dice.
At 19th level, they cannot do the same, since they only have 15 levels in Thief. They cannot gain a 17th level multiclass ability.