Wait until the avocados are soft, like a soft peach under the warty green skin. Then mash them with a fork or puree them in a blender. Once the avocado is opened, it will brown (oxidize) unless you add some acid (like lemon juice), but the browning is only cosmetic, not harmful.
Any particular reason why you're starting solids at 5 months? Most recommendations are to wait until 6 months.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"