By default you will have two stages only BACKLOG and DONE as shown in the image below
New stage can be added by adding stages=<giteaLabel1>:<stageLabel1>,<giteaLabel2>:<stageLabel2> query param in the url and that will map the giteaLabel to stageLabel representation in the kanban.
Gitea Kanban has four filters repos, assignees, labels, milestones by which you can filter the issues view as needed. By selecting one or more filter, the URL will be changed accordingly.