First choose your style -- whether you'd like a Crop Top (up to 16 inches measured from the shoulder), Full Length (up to 22 inches), Extra-long (up to 30 inches), or a Sports Bra. Once you know that, just head up to the top left navigation links and choose your particular category where you'll then get to choose your fabric from one of the many listings.
Inside each listing you'll find different colors and designs to choose from, and at the very bottom, a measurement guide. Use the measurement guide to take your measurements as directed and note them down somewhere -- you'll need them in a moment! When you have your measurements, choose your color option from the drop-down menu, the number of garments you want, and click "Add to Cart." When you hit that button, a form will pop up for you to insert your measurements and make any individual requests (narrow straps, wide neck, no hip flare, relaxed fit, etc).
At this point you can either decide to check out, or you can continue to the associated Add-ons listing where you can purchase additional customizations (a zipper, racerback, lace overlay, etc).