Once you have the Developer ribbon enabled, you can open a new Outlook item (of the type you want to design) then choose Developer ribbon, Design this form and the form opens in design mode. Any content already in the Outlook item will be carried over into the form.
If you prefer, you choose Design a Form and browse for the form type you want to design.
Design and Publish commands on the Developer ribbon
In Outlook 2007 and older, you can open forms from the Tools | Forms menu, choosing Design a Form when in the main Outlook window.
In Outlook 2003 and older, you can also use the Tools | Forms | Design this form command in an opened item.
Creating different read and compose pages
Outlook supports separate read and compose pages. Check the settings under the Page options and if you are using a separate read layout, you'll need to Edit the Read Page too.