Setting the saxophone reed correctly is extremely important since the sound is produced by the vibration of the reed and the reed must be aligned properly in order to get the proper vibration necessary for good tone production on the saxophone.
You should have the reed soaked in water and ready to be played or tested for playing. Take the reed and place it onto the mouthpiece with the tip facing up against the tip of the mouthpiece and line it up with the top edge of your saxophone mouthpiece. The tip of the reed should be exactly at the tip of the mouthpiece.
Now check to see that the bottom of the reed is even from side to side on the mouthpiece. When you are doing this you should be looking at the reed/mouthpiece at a 90-degree angle. Your line of sight should be directly horizontal to the setup you are working on. Then you can see the tip lined up correctly as well as the bottom of the reed and the sides.
Take your ligature and carefully place it on the mouthpiece while holding the reed in place with your thumb of your other hand. Usually, you have to switch hands to hold the reed in place at the top when your ligature slides down towards the bottom of the reed.
Sometimes there is a line on the mouthpiece as to where the ligature should go but it is usually just below where the curved part of the mouthpiece ends (this is the part that you place your teeth onto when playing) and starts to go straight down.
Tighten the screw or screws, or whatever mechanism is used on your particular ligature to tighten a seal. When it is tight, check the top, bottom and sides of the reed against the mouthpiece to visually check if everything is evenly placed.