Stand in front of the mirror and hold the mask up to your face.  Make sure that the top portion on the mask is on the bridge of your nose but not in your eyes and the rest is touching your skin, not hanging past your chin or covering your nares or corners of your mouth.  If it is, it's too big/small.  If the mask has an adjustable forehead/bridge piece,  adjust it so that the mask touches the bridge of your nose and the forehead piece barely touches the forehead.  Now, go in your bedroom and lie in the position in which you sleep most.  Put the mask on loosely, but don't connect your pressure yet.  ALWAYS start with the top straps.  Adjust them so that the forehead piece is touching the skin but not so tight that the mask wants to embed itself into the bridge of your nose.  Then do the same thing with the bottom straps.  Don't worry, most masks have some sort of exhalation valve on them for safety--you can keep breathing    Finally, add your pressure.  If you feel a leak, immediately unhook your pressure and lift your mask straight off of your face.  Set it back down making sure not to do the "squishy-squishy" as I like to refer to it.  You all know what I'm talking about-don't pretend you don't!  When we move the mask around once it is touching our face, the seals on them become compromised.  Any leak that we temporarily fix will return later.  Usually that occurs when we are having the best dreams possible and we are forced out of it by a steady stream of air blowing in our eye at 2am.  Once you have set the mask back on your face try hooking up your pressure again.  This time if the leak is still there, try tightening the mask a little bit-remember to start with the top.  Same rule applies here.  If a massive leak occurs, unhook pressure and try again.  Once you have gotten it to seal, turn you head in another usual position.  It shouldn't leak when you move around.  Finally, if the mask is still leaking, is too tight or you wake up with headlights or a square imprint on your face that is sore after a couple of nights--call your home care provider and tell them you need a refit.  If the mask is fairly new (couple of days), tell them that they should pick up the cost of the new mask because you were fit improperly by their staff.  If the home care providers are held accountable for their usually hurried approach to fittings/set-ups, maybe they will take more time with the patients.  Don't get me wrong, not everyone that needs a full face mask can be properly fit with one.  But there are always options and alternatives.  If they say that they don't have anything else-do your own research.  You may have to purchase it yourself, but if it works it works!

Hope this helps!