A concrete practice-tool routine for Nunu & Willump, built around its real cooldowns and ability costs.
Nunu increases the attack speed and Move Speed of Willump and a nearby ally, and causes Willump's basic attacks to damage enemies around the target.
Do nothing else for five minutes. The passive is the piece nobody isolates, and it is the only part of the kit that is always available.

Nunu increases the attack speed and Move Speed of Willump and a nearby ally, and causes Willump's basic attacks to damage enemies around the target.
Willump takes a bite out of a minion, monster, or enemy champion, dealing damage and healing himself.
Willump creates a snowball that grows in size and speed as he rolls it. The snowball damages and knocks up enemies.
Nunu throws multiple snowballs that damage enemies. When he's finished, Willump roots any champions or large monsters that were hit by a snowball.
Nunu & Willump create a powerful blizzard in an area that slows enemies and deals massive damage at the end.
Practice tool lets you reset cooldowns, which is useful for learning a combo and useless for learning a champion. Turn the reset off and cast on the real timings — knowing what an ability's downtime feels like is most of what separates a comfortable champion from a new one.
Nunu & Willump is mana-gated, which is the real limit on how often you can contest a wave early.
Unglamorous, and the highest-value five minutes of the routine. Melee champions have completely different tower-farming patterns, and this is where you find out which one you are actually playing.
These are the designers' words, not ours — the tips shipped in the client for Nunu & Willump.