idApi.root is an OPML Editor tool that implements the server side of SLIde.
SLIde requires a whole domain, not a sub-domain, because it will, in the future, allocate a sub-domain for each user. So the first thing to do is to register a new domain if you need to, or decide which existing domain you're going to use for your server.
Map that domain to the server you will run idApi.root on.
In the OPML Editor, choose Tool Catalog from the Misc menu, and click on the Install link next to idApi.root. Click on OK in response to all confirmation prompts.
Once the tool is installed, choose Add Domain from the idApi sub-menu of the Tools menu. Enter the name of your domain and click OK. It creates config.idApi and sets up an initial domain for you. The users for this domain are the users in the default table of members.root on this server. If you were running apps that use membership before, the same members will be available through the idApi.
Then set up each of the servers that you want to run off this identity server, following the instructions in the next section.