Yeah, it's on p. 282, but the marginalia on p. 283 go into more detail. The basics are:
1. Take yourself Out with the magic. Since you won't be using Consequences to avoid this, it's not hard.
2. Spend Fate Points or Tags you didn't use on the spell to pay for points of powers on a 1 for 1 basis.
There is no three.
As an example, say you want to turn into a Hawk. You want Beast Change, Diminutive Size, Claws, Wings, and Inhuman Speed. That's 6 points of powers.
Now, say you have Good Lore and 4 Stress Boxes. You'll willingly fail your Defense roll, so you only need about Complexity 5 to do this, so you do some prep-work, and accumulate a single Tag, which you'll use to boost Complexity. But you don't want to spend 6 FP on it, so you then go on to accumulate 5 more Tags to help pay for the form's powers. You do the spell, use the 5 Tags and 1 FP to pay for the powers, and you're now a Hawk for a while (one scene by default, hough you can add duration onto that by increasing Complexity).
While not an official rule, iago also said a while back that it would be reasonable to pay for powers with the temporary loss of other powers, so if you can't do Evocation as a hawk, you might need three less Tags/FP. That's at GM discretion, though.