The second warmup in my sequence is a tongue roll or trill, which is similar to the Italian or Spanish "r." I usually begin on low A and do the arpeggio pattern, 1-5-3-8-5-3-1, or do-sol-mi-do-sol-mi-do, up to at least high F. You should drop out, switch to falsetto or head voice, or drop down an octave if the exercise begins to feel tight or if you feel discomfort. Many singers find this exercise a little more difficult than the lip roll.
Here is a link to an mp3 I made to use as a background in order to practice.
Download an article, "Lip and Tongue Trills--What Do They Do For Us?" by voice scientist Dr. Ingo Titze. [NOTE: Archived here.]