When Avocado's are ripe, I cut them in half, de-seed them and put each in a quart size freezer ziplock bag. I then put about 1 T. of fresh or bottled Lemon juice in bag with Avocado. push out as much air as you can with out crushing Avocado and close. I turn the bag a bit to try and get all the Avocado covered in lemon juice. Then put the bag into the freezer. I estimate it will stay fresh for about 1 months.
You could mash them and mix with the Lemon juice to.