You start with using Business Manager at Business.Facebook.com.
There are two primary ways to manage multiple client accounts and both require that you have your own business manager.
The first way which is the considered a “best practice” by Facebook is to as for access to your partner’s accounts through business manager. (LINK)
When using this method, you will work directly inside of the client’s account on their behalf. The client will connect a payment source to this account. They pay their own bills for adspend and can put a cap on how much can be spent. You only have to bill for your services and do not need to include adspend in that billing.
Because you are using Business Manager, you will be able to control the ad account and pages as well as other users if you have other employees in your agency.
For many people just getting started in the business of running Facebook ads for clients, this is a good way to protect yourself, so you don’t get stuck with the bill.
The upside for the client is that they control, payment, own the data and creative at the end of the campaigns.
As a provider, understand that the data belongs to the client in this case.
The second primary method is for you as a provider or agency to set up a client account that you own within your own business manager. With access to up to 1000 accounts in each Business Manager, you have plenty of room to do this.
Adspend payment can be covered a few different ways. You can have the client put in their payment information or you can simply take their information and input it. Alternately, you can use your own payment method and bill the client for the spend as well as the service fees. The upside to using your own billing is that the you can use credit cards with points (A common one is AMEX with offers 3x points on advertising) which is like a bonus to the base pay plan.
The upside to this for the agency is that you control and own all the data. The client is simply paying for results. When you (if) sever the relationship, all the data, copy, imagery remains in your control.
This upside for the agency can be a downside for the client. I’ve seen cases where disagreements happen and clients loose control of their own advertising and pages, basically because they never had control.
What I use:
I use the first method with clients in my agency. My belief is that they are paying us for the data as well as execution.