To create or edit a ladder, go to the "Ladders" section of the "Start Organizing" menu at the top of the page. Next, choose a group you want to add a ladder for. If you want to make a new ladder, click the "Add New Ladder" button to get started adding a new ladder. Otherwise, click the manage button next to a ladder you want to edit, then the edit button on the next page to edit that ladder.
On the editing page you can give you ladder a title (this is for your internal use only) as well as write internal notes about the ladder, explaining to your colleagues what this ladder does. Below, you can click to start creating new rungs, triggers, and steps and build your ladder if your ladder isn't published yet. If it is, you can't add or remove rungs or steps, but you can edit existing ones to add or change triggers or edit step details. Most step detail edits will apply to everyone in the ladder or everyone who matches trigger conditions going forward except for waits -- if an activist is already in a wait when you make an edit they will not receive a new wait time, but will rather proceed with the old one. Any new activist entering the wait will receive the updated wait time.
In the toolbox on the bottom, if your ladder is not published yet you can save your ladder as a draft of publish it, and if it has been published already you can save your changes.
See here for more information on the components of a ladder -- rungs, triggers, and action, decision, and wait steps.