First, find a location that can handle a couple of dozen people and will let you return on a regular basis. You may not get that many to attend, but it's better to have too much room than not enough.
Churches, YMCAs, or community centers work for this purpose, but you may have to pay a modest rental fee. Our jam is held in one of the classrooms at a church. They don't charge us (a member of the church is one of the regulars), but we always leave a "love offering" for the use of the room. Each person who contributes usually throws in $5 or $10. Renting space may cost you $50 or more.
After you reserve the space (you've had to set a date by now) advertise on MeetUp or Craigslist or in a local weekly entertainment paper. Encourage people of all ages and all abilities to attend. Ask them to bring music stands if they have them. Set aside a couple of hours for the meetup. You may spend most of your time introducing one another, but that's OK.
Then, put together songs to play. You might want to use "The Daily Ukulele" or just put together a couple dozen songs you've gotten from uke sites, make copies, and put them in binders.
And then make it a regular event. Ours is the first and third Thursday of each month, 7-9 p.m. Once you get some consistency in the schedule, more people tend to come.
Those are the basics. I'm sure others can offer more detail.