Chop the rhubarb finely to expose maximum surface area -- I like to pulse it a few times in a food processor. Place in a glass jar, cover with grain alcohol by an inch or so, screw the lid on, and allow to steep 2-4 weeks. Over this time, the flavor and color will leach out of the rhubarb, leaving the alcohol rosy and the rhubarb a sickly yellow-white (the exact amount of time this takes will vary).
When the rhubarb has finished steeping, strain it from the alcohol, and filter the solution through several layers of cheesecloth or, preferably, coffee filters. Measure the final amount of alcohol -- this is your base number. In a saucepan, heat 1.5 times that amount of water, and 1/2 - 3/4 that amount of sugar, depending on how sweet you like things (I tend towards the middle). To give an example: 4 cups rhubarb alcohol would need 6 cups of water and 2-3 cups sugar. Let the sugar syrup cool, then add it to your filtered alcohol. Taste (the flavors will be a bit harsh), and add more sugar if desired. Let age for at least a month before enjoying. Rhubarb liqueur keeps at any temperature, but is especially delicious straight from the freezer.