Simply hang with both hands on the same campus rung for the recommended amount of time.
Do not crimp hard or use your thumbs. Use an open-handed crimp grip and let your thumbs just be bent, not grabbing or pinching the rung.
Make this easier by using larger rungs and/or putting your feet on holds.
Make this harder by using smaller rungs and/or attaching weights to yourself.
This video is used in a rock climbing training program available at http://www.trainingbeta.com