1) Create a committee or get a few commitments from members to help, especially with set up and clean up.
2) Determine how important it is to keep costs down.
3) Determine how important it is to make the menu fantastic.
4) Estimate expected attendance.
5) Discuss your plans with the appropriate Shul officers.
6) Call caterers that can meet your needs and discuss pricing for varying options.
7) Select caterer and menu and agree on a price.
8) Get the menu and the price in writing from the caterer.
9) Advertise lunch to members. Multiple forms of nudging/communication will be necessary.
10) Make it as easy as possible for members to say they are attending.
11) Have the committee members encourage other members to attend.
12) Use software or keep a spreadsheet of attending members.
13) Create a table layout and seating plan if necessary.
14) Make sure the food is delivered on Friday.
15) Check that everything agreed upon was delivered.
16) Set up the warmers.
17) Prepare the tables before the lunch on Shabbos morning.
18) Plan out the timing of when the courses will be served and any speeches will be given.
19) Keep the timing reasonable.
20) Clean up the room properly after the meal.
21) Give the treasurer the amounts to be billed for the attendees.
My (Conservative) shul does lunch every week. When a congregational lunch happens every week, the cost is built into the yearly budget (although there is a big push for small donations to commemorate each and every birthday and anniversary), the attendance is predictable, the caterer works as an hourly employee of the shul & can pick menus around what is in season and/or on sale, and so forth. This really builds a Shabbos community when not everyone who comes to a shul walks there (I’d say about half of us walk); we spend a large portion of the afternoon together every Shabbat.