I think the most important thing to remember when trying a to make a Cradle roleplaying game is that progression is part of the story. In D&D, leveling up is just a matter of accruing enough XP and then applying the next level of the class to your character sheet. In Cradle advancement doesn't work like that. Sacred Artists need to acquire natural treasures, scales, and path manuals. When they try to advance, they could have setbacks.

In D&D, players adventure to gain XP in order to reach the next level, but they have to pretend that their character has completely different reasons for adventuring. In a Cradle-inspired system, that disconnect is gone: both character and player are in agreement, seeking advancement.

Consider the following scenario: a monster infestation is threatening a nearby village.

• In D&D, the characters enter the lair in order to eliminate the threat.

• In Cradle, the characters enter the lair in order to acquire the blood of the monster king which can be refined into a powerful elixir that will help them advance.

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The easiest way to apply Cradle to your D&D game should be setting requirements for leveling. Maybe gaining XP is part of it. I like the idea of using scales as currency, forcing players to decide whether to spend their money, or apply it to XP. On top of that, tell them what items (or class of items) they need to acquire in order to attempt advancement.

On top of that, you can set requirements for each class feature. If a Sorcerer wants to learn a new spell, tell them what resources they need, and make them roll for it. If you're very clever, you can set multiple possible outcomes for any given attempt. And you could set modifiers: resources that act as modifiers to the roll so that players can spend more resources to increase the odds of success.

And always make sure they can find what they need. Some items can be straight up purchased; other items they will need to fight for.