Decide where your editorial calendar will live. Possible options include Google Calendar, Podio, Trello, and Asana. Each of these systems is great for multi-user management, and all of them have free versions.
2. Start building
Start with a calendar that marks important dates. For instance, if I'm a romance writer, I'll be sure to include Valentine's Day. National holidays are a great start, but don't forget about silly "national day of ____" ones. Also include any publication dates you're aware of for articles coming up.
Next, figure out your goals for publication. Whether you publish 5 articles a day or 12 per year, enter those slots into your calendar.
Now you have the skeleton of a plan. Fill in each slot with proposed author and topic, and work backwards to fill in deadlines. If you know you need a week of writing time per article, set deadlines for content a week prior to each slot. Set reminders for yourself to make it easy to remember.
How far in advance you plan is completely up to you, but I'd recommend at least a few months into the future. Many publications have an idea of their ed schedule a year or more in advance. Examples here.
If you manage more than one section, I'd suggest separating them into multiple systems. Most of the suggested systems above make that easy (ex: different Trello boards, separate Google calendars).