When sequencing RNA other than mRNA, the library preparation is modified.  The cellular RNA is selected based on the desired size range.  For small RNA targets, such as miRNA, the RNA is isolated through size selection.  This can be performed with a size exclusion gel, through size selection magnetic beads, or with a commercially developed kit.  Once isolated, linkers are added to the 3 and 5 end then purified.  The final step is cDNA generation through reverse transcription.