I've heard of using a sand/gravel mix but in my opinion you might as well just use one or the other, if you want to have part sand part gravel the sand starts to mix with the gravel over time (how long depends on your fish and how careful you do water changes) one way around this is to use a piece of perspex or similar or border off a section of tank, if its cut just above the line of substrate you shouldnt have many problems or there pure sand or pure gravel of course
Step one, choose how you wish set up sand and or gravel
Step two, wash sand VERY well
Step three, remove all fish and water from tank
Step four, - if using sand/gravel mix, remove 50% gravel, mix in your sand
-if using other options remove part or all gravel, add sand
Step five, using a bowl or similar resting on tank bottom (this helps minmise water disturbance) fill tank about three quarters, add plants, rocks etc, fill tank rest of way
Step six, turn heater/filter back on
Step seven, after water clears add fish
HTH
Andrew