Start on one side of your mouth, using your fingertip on the inside of your back molar (furthermost back tooth) slowly pull the aligner from your molars/teeth.
Do the same thing on the other side of your mouth before completely removing the aligner. A little bit each side before the whole thing!
Once the aligner is free from the molars on both sides of your mouth, you should be able to slowly work your way forward, gently pulling the aligner away from your teeth with your fingertips.