Come on groundup! Everybody knows that!
Here's what I do...
Open your blank capsule button
Open your new text/icon
[text/icon] Press Ctrl A, then Ctrl C to select all + copy
[capsule] Press Ctrl V to paste as a new layer
You should now have a capsule and a text/icon layer
Press Ctrl + three times to zoom in 400%. It's easier to work with this way.
Right click on the titlebar of the image, then select "Canvas Size". Choose a new width that will accommodate your new text.
In your Layers palette, select the capsule layer and then check the box to the left of the text/icon layer to temporarily link these layers together. Press V to select your "Move" tool. Use the left arrow key on your keyboard to move both layers all the way to the left of your blank canvas.
Press M to choose the "marquee" tool
Make sure you have the square "marquee" tool selected. If you do not, press Shift M repeatedly to cycle through the different shapes of "marquee" tools.
[capsule] Select the far right portion of the button and press Ctrl and the Right Arrow on your keyboard to move the button "piece" further right to make room for your text.
[capsule] Now select a center portion of your button and press Ctrl, Alt, Right Arrow to "stretch" the center of the button to meet the end "piece".
Unlink the two layers by clicking on the small chainlink icon in the layers palette. Next, select your text/icon layer and press Ctrl and the Arrow keys to position the text and icon where you want.
Press Ctrl, Shift E to merge the layers into one image. Press C to select the crop tool and crop only the remaining blank canvas space in the width of the button. Leave the blank space remaining in the height to make sure all your buttons will line up properly on your board.
Press Ctrl, Shift, Alt, S to save to the web as a gif, 64 or 32 color format to make the filesize smaller but retain graphic quality.
That should be it. Let me know if you have questions or if there's any typos...