A nicely ripened avocado will have a dark skin and will be slightly soft when pressed.
Now, this is how I like to cut them.
Start from the top. Find the seed with your knife and cut around it.
Twist to open and remove the seed with your fingers.
Score vertically and horizontally.
Invert, and remove the pieces with your fingers or a knife.
I dressed my avocados with salt, pepper, and lime juice. I really recomend adding halved grape tomatoes to this and eating it with corn chips.