Set up your first meeting and invite all interested neighbors. You can make flyers to stick under doors or in common spaces. However, beware that the landlord could find out what is happening before the meeting occurs. Don’t invite your landlord to the meeting, or the building manager. It’s best for tenants to get to know each other without the pressure of a building manager present.

Make sure your meetings have a clear goal and a facilitator or someone else who can guide the group towards that goal.
Write down the main decisions you made in the meeting, and send them out to everyone. It’s often easiest to have someone take notes on these while the meeting is happening so you don’t forget.
Be aware of group dynamics.

Bring some snacks and take time before or after the meeting to chat with your neighbors a bit. Folks who know each other better get along better and are more resilient in the face of backlash from the landlord.

Handle all conflict openly, immediately and with respect.
For example, if one person talks the whole time at the meeting, it can be a big turn off for others. You can solve this by having people raise their hands while the facilitator calls on them if it is a big group.

Decide when to organize publicly and when to exercise secrecy. The sooner your landlord knows you are organizing, the sooner they can retaliate against you. That being said, if you can prove your landlord is retaliating against you for organizing you may be able to defend yourself using California Civil Code 1942.5.