Determined to figure it out, I sat down today and in a few minutes I got it done. Here is how I did it! All my work is done in Photoshop CS6 on a Mac, fwiw. If all you want to do is to fill with a solid color, you can quickly do so with the paint bucket tool. Click on the color picker, choose your color, and click with the paint bucket (shortcut=G) in the white areas of the letters to fill:
But, this doesn't help if I want to use patterned papers to fill the letters. That takes a few more steps. First, you need to rasterize the font layer. I prefer to always make a copy of the layer first (cmd or ctrl J) just in case I need to go back to the beginning and start over without completely starting over...once you rasterize a text layer, you can no longer go back and edit the type. To rasterize, I click on the layer in the layers palette and choose rasterize. Now, if you turn off the eyeballs next to all the layers but the new one, you will see that the black outlines are on a transparent layer, like so:
You will need a new blank transparent layer at the end of this next step. Just click the new layer icon at the bottom of the layers panel so it is ready. Now, cmd+click (or ctrl+click) on the thumbnail of the rasterized text layer and it will put marching ants around the letters. If we filled now, it would just change the color of the currently black parts of the letter. We could have done that when typing the text to begin with, using the font panel color picker. To now select the inner parts of the letters, cmd+shift+I (ctrl+shift+I) to inverse the selection. You'll know it's inversed if the marching ants are around the whole screen. Now click on that blank layer we made and click inside the letters with the paint bucket. It will look like this:
Don't despair--it is easy to get rid of all that extra color. After hitting cmd+D (ctrl+D) to get rid of the marching ants, find the magic wand tool. Be sure the contiguous box is checked and click anywhere outside of the letters, then hit delete. Voila!
Now, turn off the black outline layer and you will see your clipping mask:
And when you bring in some cute patterned paper and option+click (alt+click) between the layers in the layer palette and clip the paper to the mask, you can get something like this:
(Bella Gypsy Road Trip Multicolored Chevrons paper)
It is completely reusable, as long as you always want the same word. Or, if you want to make your own alpha set you could fill a page with the alphabet and follow the same steps. And then just save as a PSD or TIFF to be able to change the background at your will.
BONUS: Make it a digi sticker!
On the outline layer, add a stroke in layer effects. Make it white and a width of about 30 px and you can have this (with a blue background layer to see the stroke outline better):