Hi
I have quite a lot of experience in setting up and running PMO (as a part of performance improvement and turn around projects)
In short what you need:
1. Agreed with your boss framework in which projects will reported to you and steering committee or other supervision body. You need also some tool to help you with seeing where you are and making reports. I used Excel (with VB macros) and Google Sheet. For more advance solution you can use Asana, SmartSheet, Lean Kit,
2. Agree on how projects should be run – how often you meet with PM, how often PMs are meeting with their team, how team leaders are chosen, how resources are allocating to the project and who decides on this; what is the reward / motivation scheme for PMs and team members
3. Communicate it to the project leaders / stream leaders – I proposal to make a training and presentation available to everybody – record it so that you do not have to answer stupit questions time and again
4. Execute and improve
That is in short
If you want to have a look at the excel just write to me

Best regards
Asen