Allied and affiliate associations of the MLA (such as WiG), are allotted a certain number of guaranteed sessions at each Annual Convention. In addition, any MLA member may propose a special session. Such proposals are evaluated by the MLA Program Committee, and about 1/3 of submissions in any given year are accepted.
I recommend that you begin planning a special session proposal around 18 months before the session is to take place (one year ahead of time is almost too late!) At all stages of the process, refer to the MLA's official guidelines, which are very specific. Up-to-date guidelines for proposers/organizers of special sessions can always be found in the September edition of PMLA, and on the MLA's website: www.mla.org.
You can hand-pick your panelists, or solicit proposals via the MLA newsletter and/or any other channels, such as listservs. If using the newsletter, check MLA deadlines (usually early September for the winter edition and very early January for the spring). When contacting prospective panelists, make sure you advise them that a special session is only a proposal at this stage, and must be approved by the MLA program committee in order to take place.
When you select your panelists, inform them that they must be MLA members by April 7th in order to participate in that year's convention (waivers are possible - check the guidelines). I have also found it helpful to warn prospective presenters that the MLA provides no funding for presenters.
Once your panelists have agreed that - should your proposal be successful - they will participate in the session, you can now get to work on the proposal. The MLA guidelines include suggestions as to how to maximize your chances of success. Use of the MLA's official special session proposal form is strongly recommended! Check also the submission deadline (usually April 1st).
The Program Committee meets in early May to choose special sessions, so you should hear by early June whether your proposal has been accepted. Don't forget to inform your presenters of the outcome! If your proposal is successful, you will receive further instructions from the MLA as to how to proceed.