Tower diving is the risky move of attacking your enemy under enemy tower. It is extremely important to decide on the correct time to dive.
How to Tower-Dive
First, you need to have a good sense of enemy cooldowns and his escape techniques and crowd control abilities. Diving on an enemy with a flash is almost a suicide. As a fiddle player, I walked with double kills using the fear, silent and drain on so many occasions. If you are enemy is one hit away, I always recommended a tower dive (ya! even for Kassadin). However, do it in a way that surprises your opponent.
Always keep your CC to use on the correct time. Tower diving requires a CC on most occasions. If you do not have a good enough CC always get the support of your jungler and Mid (If you are mid, take help from bot/top lane). Good jungler always has a good gap closer and CC to help you.
If one of you is low on health, just get away from the tower after doing the initial combo. Your friend will finish him off. Remember, both sides getting 1 kill each is not a good tower dive.
Always ward for enemy jungler, they normally sense the dives. If the enemy has flash do an initial fake dive. Otherwise, keep your gap close till he burn his flash.
Countering the Enemy Tower Dive
Above all, ward! Normally people gank as groups, make sure you know when enemy jungler is approaching.
If you think you can come out ahead from being tower-dived, do not just ping the fog-of-war where the ganking enemy lurks. Tell your team you are about to be dived, spell out exactly what you need from them, and then ping. Ex: “About to be tower dived. Soraka, be ready to heal.”
Dodge enemy skill shots. Specially ones like Lee Sin’s Sonic Wave. Move randomly and, do not be too close to walls.
End games dives are devastating; just do not think tower can protect you if you are low on health. Only thing that can save you is your fellow team members. If you do not have and help near you, just back off.
Always make sure that you hit at least one for all dying enemies. Do not let him get executed by tower.
As the enemy tower dives, save your summoner spells like flash or exhaust and any crowd control ability until an enemy gets tower aggressive and make sure it is not a fake dive. The number one goal when tower-dived early-to-mid game is to prolong your survivability while the tower does maximum damage.