the process is pretty straightforward, as jonny said.
basically, you select your parts on saberparts .com, and keep your parts list handy. i'd save a copy of the url that appears when the complete saber is designed.
then you go to the indiegogo page (as linked above) and select your tier, whether it's warrior- or champion-tier, etc. pay via credit card. you'll receive a pledge number.
you then email sales@saberforge .com with that number and your parts list. i'd include the url as well, but that's just me.
you'll then receive a confirmation email from customer service with your usps tracking number and the wait begins. there's a 7-14 business days window where your saber is built and shipped.
you can check the progress of your package on the usps website or via a usps app. you can also opt to receive email updates.