ImageReady

With the layered document active, go to File> Edit in ImageReady. (Note: You can switch the document between Photoshop and ImageReady as much as you like using the same method. Once in ImageReady, you will have the File> Edit in Photoshop option).

ImageReady will open up and you will see a similar screen to the Photoshop one with an additional 'animation' window as well. (If you can't see the animation window floating around, select Window> Animation from the top menu.)

Start by adding frames to the animation window by clicking on the little arrow on the top right and selecting the 'New Frame' option on the fly out menu (as shown below), or click the page icon at the bottom of the animation window. Add as many frames as you like - you can also add and delete as you go along. It is important to note though, that the more frames you add, the bigger your file size is going to be.