I measure low freq and hi freq in two different ways. First, move your listening chair or couch out of the way, but mark on the floor where your ears are when you're sitting in it.
Mount the SPL on a short (table-top) tripod adjusted so its mic is between 1 and 2 feet above the floor over the listening position, with the SPL mic pointed toward the ceiling, like Kal says.
Hi freq. (350 Hz (+/-) and above:
Mount the SPL meter on a normal tripod adjusted so its mic is at seated ear level (about 40") over the listening position and pointed forward between the speakers. You may have to rotate the tripod head 90 deg. sideways so you can read the meter from a few feet away without standing on a ladder ;-))
I recommend not moving your chair, as it's absorption/reflection will affect the readings. You want it where it will be when you are listening.
Put the meter on a tripod exactly where your ears will be, and pointed between the speakers or slightly above so you can read the scale.
Be aware that moving your body up to make readings will also affect the readings, so try to read them from a consistent position as far back as possible.
Tboooe, as in the Article Fishboat suggested, bass (starting at, and below 400 Hz) is non-directional. It appears to come from all parts of the room at once. So placing the mic about a half wavelength of 400Hz from the floor (about 1.5 feet) it will "see" the room in all directions from your listening position.
Above 400 Hz, sound becomes more directional. And of course, the speakers themselves are (usually) designed to spread (or "disperse") the sound in a predetermined pattern. This causes the two channels to interact with each other and the room in very specific ways. And depending where in the room you are going to sit, and which way you are facing, you will hear a certain "response curve" which is happening at just that spot. So you orient the mic to takes the place of your head/ears.