In addition to this page, kalith created a tutorial video. And here's another tutorial by Stewiejp.

The first screen you are presented with after you press that "New room" button should look something like this:

One of the first things you'll want to do is:

1. Select a map.
2. Copy the link by pressing this button or just copy/pasting the address url bar.

You can then share the link with your friends or clan mates so they can join your tactic, either so you can explain it or so they can work on it with you.

After selecting a map and people have joined.

3. List of users that have joined, click on their name to toggle their privileges (yellow means can edit, green means can edit and give permissions to other users).
4. Alternatively you can give everyone edit priviliges by unlocking the room.
5. You can now start creating or explaining your tactics with the tools here, read the tooltips.
6. The options of the tool you've selected will appear here.
7. You can chat with the people in the room here.