How do I fill outlined letters?

I will try to provide to you more precise instructions for the same process:

• Create type layer using Horizontal Type tool.
• After creating text go to File > Place Embedded and navigate to image which you want to use.
• Drag placed image above text layer in the Layers panel.
• Ensure that image layer is selected then press Ctrl + Alt + G that will do the trick to see image inside text.
• Select text or type layer in the Layers panel then go to Layer > Layer Style > Stroke. 
• Set Postion and Size, Color etc for Stroke effect then click OK to confirm and exit / see previous post and screenshot.

Create your text layer. Add your image layer above it and hold alt/opt while clicking between the two layers in the layer panel to clip the image layer to the text layer. Then double click on the text layer in the layer panel to bring up the layer effects dialog box. Add a stroke to the text layer.