The process is easy, pick your blank, buy your components and build it LOL!
Seriously it's no big deal, the only thing is many warranties are voided when you do a float rod build on a spey blank.
Sage is one of the very few that will still honour their warranty when you do this.
If you need help deciding on exactly what components to use feel free to pm me.
In many cases blanks and components can be purchased online easily and cheaply..
Speaking of a 121/2' 7wt, here's mine: Sage 6126, 121/2', 6/7wt.
I had it built with oversized S.I.C. fly rod guides so that I could throw my centrepin on it and switch from flyfishishing to floatfishing as the mood strikes. (fantastic rod btw)