Cut in the frozen butter or just dig in and mush it up, but you have to work very quickly. You DO NOT want to melt your butter or make it soft. It is part of what makes them rise. Make sure everything is cold you work with.
This is where I made a change. I grated the butter then tossed it in the flour, turned on the beater for about 30 seconds added the wet ingredients and the berries and then once almost all was wet about 45 seconds dumped it onto a cutting sheet. Kneading it about one minute then formed my flat round and cut it. It was faster and easier.