I'm just about to do this myself. My method:

Under "Practice" go to "Drills"

To the far right and under "Drilling Mode", select "Use Advanced Builder"

Once you're in Advanced Builder, I've been selecting the All Prep Tests (101-158) Button + either the RC or LR button

Then I select the test + section + all the questions in that section (by hitting the little plus sign button on the far left) so that you see the blue bar of them selected

And then hit the Create Drill with 26 Questions that appears in the bottom right.

Good luck! Happy drilling!