Determine the point at which you want the other elements to drop out. This will usually be at the end of a verse or chorus that has some sort of dramatic significance to the rest of the song. Starting the breakdown immediately after a crescendo or dramatic peak will create the most startling contrast, emphasizing its effect.
Work through the song until the next big "drop," usually the start of a chorus or an instrumental section. This will be the point at which the breakdown ends and all the other instruments return. Count or measure the distance between the start and end of the breakdown; it should be a musically appropriate number of bars, eight, 12 or 16 are the most common choices.
Divide the number of bars in your breakdown section by the number of instruments in your track. If you have a four-part track, consisting of drums, bass, guitar and synthesizer, and a 16-bar breakdown, each stage consists of four bars. Breakdowns almost always use these progressive stages of equal lengths to raise or lower the complexity of the song.
Starting from the beginning of the breakdown, select the first four bars and delete or mute all instruments but the drums. Move to the following four bars and delete all but the drums and one other instrument from the rhythmic foundation, usually the bass. Repeat this process, gradually bringing in more and more instruments. You may wish to substitute some of the parts for harmonized versions or alternate rhythms to help increase the tension.
Help hammer home the dynamic shift by dropping out all the instruments but the drums in the last bar of the section. This helps maximize the contrast before the main part of the song comes back in.