I've been growing an interest in running signature campaigns.
I understand my rank might not be the appropriate to have the trust needed to run such a service but you have to start somewhere and I figured I'd open it.
You have to grow your rank.
You have to hunt for less known small campaigns and make your offer. Try to ask much lower price than the other managers for your services (maybe offer them for free?) to have a chance to make a business of it. You'll need to have a portfolio and that's hard to earn.
Keep in mind that most campaign managers still have plenty of room/time for even more campaigns and you'll have to compete with them.