The clipping mask feature is one of my favorite and most used aspects of Adobe Illustrator!
To start off I usually use a blank canvas of 1000 x 1000 pixels for a graphic such as this.
You want to use the type tool to write what you wish and to pick the font you want.
Once you have your text the way you want it, you then want to go ahead and pull in the design you want the letters to be in.
You then want to make sure ONLY the pattern is selected, right click and hover over arrange and click on send to back.
Then make sure to have the text centered on the pattern the way you want it.
Make sure both the pattern and text are selected, right click and click on make clipping mask.
And BAM you have your very own custom clipping mask :)