In a bar of sixteenth notes in * time, there are 16 different positions for notes within the bar. The first beat is 1, the second note of the first group of four is called the "e of one" the third note of the first group is called the "and of one", and the fourth note of the first group is called the "a of one".
As with other subdivisions of the beat, the system then continues through the bar - 2, e of 2, and of 2, a of 2, etc. As with other subdivisions it is worth practicing your bass drum and also your snare drum on each of these positions until you can do them at will.
Example 46 demonstrates various note groupings on the bass drum based on an underlying feel of continuous 16th notes. It is worth practicing each bar separately at first. Then try two bars together, and finally all four bars. As you play this example, count continuous 16th notes even if they are not written, and accent the bass drum part with your voice.