The double stroke roll is R R L L R R L L — two strokes per hand before switching. The trick: both notes within a hand should be equal, not loud-soft (a "diddle"). Use a controlled rebound rather than two separate full strokes.
Most beginners can play a clean RR LL slowly, but it falls apart when sped up — the second note of each pair becomes weaker. The exercises below use accents on the first of each pair to expose this weakness, then on every other pair to even it back out.
Exercises
1 — Eighth-Note Doubles
Sticking: R R L L R R L L. Slow tempo — focus on note-to-note evenness within each hand. The second R should be as loud as the first.
2 — Sixteenth-Note Doubles
Sticking: RR LL · RR LL · RR LL · RR LL — eight doubles per bar. Same tempo as exercise 1, twice as fast in the hands. If the second note of each pair gets quieter, slow down until they're equal again.
3 — Doubles with Accent on the First of Each Pair
The first note of each pair is accented (the > marks); the second is soft. The accent tells the truth about your doubles: if the soft notes disappear entirely you're cheating; if they're audible and even, the technique is real.
4 — Doubles with Accent on Every Other Pair
Now only the R's that fall on beats 1, 2, 3, 4 are accented — every other pair. The accents now mark the pulse, the same way they did in single-stroke exercise 4.