It's very quick to do this. We strongly suggest you draft your message offline on a document somewhere and then just copy and paste it into the editing box.
Rather than drafting it online.
If you have any special events, fundraising programs or it's a special time of the year, you could edit the message appropriately.
1) Log in
2) go to your Ogranization's dashboard
3) click 'Settings'
4) paste your text and check it looks ok
5) click 'submit changes'
And that's it. You're done.
Here's a quick video showing how to do this.