Cas9-sgRNA

This graphic below shows precisely where you will insert your guide sequence (target-binding sequence). The U6 promoter is colored in blue and the transcriptional start site (TSS) is marked. Notice that the TSS is just 5' to the G of the U6 promoter. Not only is this "G" necessary for pol III transcription but it is also part of the RNA! Not surprisingly, this added G does not interfere with Cas9 targeting (Recall Jinek et al. 2012). 

Your guide sequence should be 20 bp in length. It will replace the vector backbone (not colored)  If you can exploit the U6 G then your guide sequence can be 19 nt in length.