Here you can see the Drawing Board with some assignments set up.
When you are in a raid and open Drawing Board, you will get a blank board with tabs on the right side for each member of the raid. At the bottom left, there are tabs for the various raid marks. You just drag the different tabs from the sides of the board in and connect them together however you want. Aside from just having the names of raid members and the raid marks, there is an option for custom assignments down in the bottom right of the board. This option allows you to type things in to make little tabs like "heal" and "bubble everything" like you see in the screenshots.
The pop-up box to enter in text for your custom tabs.
When you create custom tabs, they will show up at the bottom between the raid marks and the custom assign button. Once you've made your custom tabs, you can drag them around connecting them to various people. If you need to switch who is healing who or replace someone, you just drag around the tabs. This is far easier than most of the assignment addons that make you delete the old name and type in the new one each time.
Another handy feature of Drawing Board is that is saves your boards. You can create numerous boards using the new board button in the top left. You can access the different boards via the tabs just under the new board button. This is great because if you run with the same people most of the time, you can have one board all ready with the assignments for the group you run with and you can have other boards for when you run with other people.
Once you have everything laid out on the board how you want, you can just click the broadcast board button up top and it will post all the assignments in raid chat.
You can see some of the assignments displayed in the chat window in the bottom left of the screenshot.
If you are co-leading with someone else and they have Drawing Board, you can even share your board with them and they can make changes. This would be handy for the times when you want a second opinion, you could just share the board instead of having to type it all out in a whisper.
I've only had the chance to play with Drawing Board on downtime during raids, so I'm sure there are other features I haven't found yet. So far, I think this addon is full of win for making a raid leader's job easier.
