The working agreement is a documented set of ground rules that the team creates and follows in order to improve their productivity and happiness.
Think of some of the things that slow your team down. For example, some teams find that their events are more productive when everyone puts away their phone and laptops unless they need them for the events. Other teams find that having individuals come late to standup limits their usefulness.
One way to create a working agreement is to use a little bit of reverse psychology. Get together as a team and brainstorm all of the ways your team can fail. Then prioritize the top 5 most significant risks to your team based on likelihood of occurrence and severity of impact. Then, come up with a specific rule that would prevent that from happening.
For instance, perhaps the team agrees that having people working on their laptops during events is a big risk to their productivity. They then create a rule that the only person who should have their laptop open is the person presenting their screen to the rest of the team. All other electronics should be put away. If there is a production issue, the person working on the issue should leave the room so as not to distract others.
See if you can come up with about 5-7 rules to help with the 5 top risks you identified. Then post it in a conspicuous location. If you are collocated, write each rule on a sticky note and paste them on a wall near your desks. If you are distributed, create a page in your wiki or similar space.
The whole team is responsible for holding each other accountable to following the working agreement. Be polite and ask each other to follow the rules that you agreed to. Escalate only as a last resort.
Review your working agreement regularly, at least once every sprint at the retrospective. The team can add or remove items to the working agreement at any time so long as there is general consensus.
If your team creates and uses a working agreement diligently, you will soon find that your team is happier and more productive. Best of luck!