Learn percussion notation. Percussion notation, like melodic notation, is written on a stave of five lines. In place of the usual treble or bass clef, a drum clef is used. The drum clef resembles a narrow rectangle set vertically. Each line and space on the stave is reserved for a particular drum voice. For example, the bass, or "kick," drum occupies the lowest line of the stave, while the snare drum occupies the third space from the bottom. In addition, certain drum voices have unique notes. For example, the cowbell is notated with an "X" in place of the standard oval seen in most melodic notation.
Establish a time signature and speed in beats per minute (bpm) for your drum part. For those new to drum notation, it is often easier to begin with a basic 4/4 signature. The bpm is less important for the purpose of writing a drumbeat, unless you intend to play the part yourself. A common bpm speed is 120, equivalent to two beats per second. If necessary, use a metronome to find a bpm you are comfortable with.
Write the kick and snare drum parts first. Just as overall percussion is seen as the "heartbeat" of a band, the kick drum is often seen as the "heartbeat" of the percussion section. The kick and snare drums together are the dominant drum voices to which the remaining voices usually act as embellishments. A common kick drum part, called a "four-on-the-floor" rhythm, has the kick drum play on each beat of a 4/4 measure, while the snare accents every second or third beat in various ways.
Notate the remaining drum voices as necessary. For example, the high-hat and/or cowbell are often used to emphasize the off-beat or to create phrases that are counter-rhythmic to the kick and snare parts. This means that they play a similar rhythm, but begin on a different beat, often the second beat or first off-beat. The various tom drums are usually used to create "tom runs" that open or close an overall rhythmic phrase. The various cymbals -- splash, china and crash -- are generally used sparingly to embellish rhythmic phrases as needed.
Ask a drummer to play your drumbeat if you are not a drummer yourself. Occasionally, non-drummers create parts that are not physically possible to play without a third arm, for example by placing a snare, high-hat and cymbal note on the same beat. A drummer will tell you if your drumbeat is viable and perhaps suggest alternative approaches to achieving the effect you want to create.
Experiment with different kick/snare rhythms and embellishments. Like with any creative process, writing for drums is a matter of taste and also a matter of trial and error. Always begin with the kick and snare drums, if any, followed by the high-hat and toms. Aside from the basic steps already described, there are no strict requirements when creating drumbeats. If it sounds good to you, it works.