Turn off the treadmill and unplug the power cord. You need to loosen the belt in order to apply/squirt lubricant between the deck and belt. Hopefully you've kept the manual of your treadmill because this will show you exactly what to do to loosen the belt of your particular make of treadmill. There are usually screws on each side of the belt that you loosen.
Lift the belt so you can apply the lubricant below it. Squirt or spray it along the whole of the width of the belt and make sure you wipe any excess so as to not stain your floor. Applying it along the width means that when you start the treadmill the lubricant will travel to all parts of the deck and motor where it's needed. Before you start it, tighten the belt using the original screws you used to loosen it.
When the screws are tightened once more, plug in and turn on the treadmill. Then allow the belt to run for 3 minutes at 3 mph. When it's done you're finished with this part of the maintenance! Remember to do this every 6 months or 150 miles which ever is sooner. It's best to do it slightly too often than too little as too much friction on the belt wears down the motor as it has to work harder.