I diluted Harleco Giemsa, original azure blend 1:20 with distilled water,
stained 30 minutes, can go longer, quick swish through 95% ethanol,
20 swishes in 100% ethanol, clear and coverslip.  If the 1:20 is to dark,
do a 1:50, you can always differentiate Giemsa, using 95% ethanol.  

You may have to try a buffer and if using a different Giemsa brand, have to
adjust times.   
                                                 
GMA was doing the blood smear method, flood slide with Giemsa for 3 min,
add pH 6.8 phosphate buffer, until green sheen appears, stain for 6 minutes,
rinse, air dry.  You can always remove Giemsa with a tidge of 100%, 95%
will take it out more rapidly, air dry and mount with xylene based media.

What is really important is a good positive control, so you can make any
staining adjustments, Giemsa can be tweaked, let me count the ways!

Gayle Callis