How can I pick the correct avocados for guacamole?
I find it super difficult to make guacamole without ripe avocados, as I like a dip with no huge avocado chunks in it. Thus, as you mentioned, ripe avocados work best. 

Ripe?

To deem if an avocado is ripe or not, give the avocado a gentle squeeze. If it is firm but gives a little (is a little squishy) then it is ripe. 

It is interesting to note that avocados do their ripening after they are harvested, so each avocado goes through this process of getting softer until it is eaten. If the avocado doesn't squeeze to your desire, time will allow the avocado to ripen more until it is the desired squishiness to be eaten. 

On how to squeeze an avocado: here

Ripening Avocados

I have found that the best way to ripen an avocado (for example you bought an avocado from the store and it isn't ripe enough for guacamole yet) is to leave the avocado in a room temperature area, by fruits like bananas, which are also ripening. 

Just like any food, putting avocados in the refrigerator will prolong the freshness of the food. Thus, refrigerating avocados won't help the avocado ripen (it does however preserve the state of the avocado, and slows the ripening period by about a week to two weeks, from my experience)  

On Ripening avocados faster: here