When putting together the clarinet, you need to make sure to not put any pressure on the keys because this can cause damage to the pads, instead, place your hands around the areas that will not be affected by the pressure (the area with the least amount of keys). Keep all of the corks lubricated with grease to insure that the parts fit nice and easily. Start assembling the pieces together with a gentle twist, when you get to the two joints, press down the key to lift the upper bridge key so you don't damage it when twisting the two pieces together, and then carefully line up the upper and lower bridge keys. .
Once the clarinet is assembled, then you can put the ligature and the reed onto the mouthpiece. Place the ligature on the mouthpiece with the screws loosened and the handles on the right side of the instrument. Hold the reed between the thumb and forefinger, slip it under the ligature until the reed is about a hair above the tip of the mouthpiece. Make sure that the reed is exactly centered and then tighten the screws firmly so that it holds the reed in place, but is still free enough to allow needed vibration.
Now hold the instrument with the ball of the right thumb under the thumb rest, with your three first fingers resting on the open holes below them, and then place your left hand on the upper joint with the left thumb on the rear side of the instrument resting at a diagonal angle on the tone hole, and the first three fingers resting on the open holes below them as well. The clarinet should be centered with the body at a 35 degree angle to the body. Make sure your head is looking straight and erect, chin up, eyes forward, with your shoulders relaxed and free of tension. Your upper teeth should rest gentle about a half inch from the top of the mouthpiece, make sure your chin is flat and firm with a slight downward pull on the muscle. Your tongue should rest on the back of your lower teeth, except when tonguing then your tongue will quickly tap the tip of your reed.