Gradually, and very pleasurably.

Naruto watched as Anko writhed and arched her back on the ground, her legs starting to sprout scales. Anko was gasping and moaning as very pleasurable sensations ran through her legs, though it also felt very hot. Almost like her very bones were melting, but not painfully. Then, she had the strangest urge to touch her feet together, like an itch in her mind that demanded scratching, and only grew more and more insistent until she gave in. The moment they made contact with each other, the skin started to fuse together, almost knitting her feet into each other, and the sensations intensified. She kicked her legs, only to find them far more flexible than they should be, and cried out as her ankles and lower legs began to fuse as well, while her fused feet extended out, squirming like a tail. The fusion reached her knees, or rather, what was left of them as they no longer had kneecaps, the bones being absorbed into her flesh as more and more scales sprouted. She clenched her fists, gouging out handfuls of the dirt as her forming tail flailed, her thighs losing the gap between them.

Then her pussy seemed to burn in a pleasurable manner, making her cry out and buck her hips as said snatch migrated upwards just enough to avoid getting lost in the melding of her legs below her hips, which had also started sprouting the scales, along with the outer parts of her boobs and arms, while she tightly shut her eyes and grit her teeth. Her legs, or rather, her tail, finished extending to be about eleven feet long as it filled out more scales until all the human skin below her hips was covered in glittering purple snake scales, with a plain white "underbelly."

The scales grew up to her hips, but parted to leave her pussy just barely untouched. Her ass crack also fused together, only leaving a bit of an indent, and a small hole for defecation purposes, not that that's ever going to get depicted.

She finally finished changing, and laid on the ground panting, opening her eyes to show they were a rich amber with slitted pupils now. She slowly rose, losing her balance initially, before figuring the tail out, and removed her headband, the ropes having broken due to her losing control of her chakra enhanced strength during the whole time she was writhing, to look at her face, which but for the eyes remained unchanged.

She put her headband back on and looked at her arms, tits and new tail, seeing the scales on the back of her hands and on her outer arms from there, as well as cupping around the underside and outside of her boobs.

"...holy shit... I look so fucking sexy," she murmured, before fixing her gaze on Naruto as he grinned at her and beckoned her over.