This guide will teach you how to get each achievement, all 14 of them. Honestly, if you’re good at the game’s driving mechanics, it should take you about less than 8 hours. Yes, I did spend two hours writing this at 2 in the night.
Finish a race in any category. No, really, you just unlock it after any type of race, just finish it. Doesn’t even take 6 minutes in most cases.
Trigger a super cannon start. Like many kart games, this one has it. When a race has the countdown at the start, hold down your driving button. It should look like you got an extra boost when you do it right.
Win a hat. Do a time trial, try getting any medal time from it. Gold, silver, bronze, platinum all work.
Win an “unique” hat. Complete a single time trial and get the gold medal time.
Win all hats. Win all race tracks on time trials on any time, any medal.
Complete the gold medal time from all time trials on all maps. Aim for the gold medal times, so you’ll need to use shortcuts and likely several tries to get it done.
Win a spoiler. Complete any Grand Prix cup on first place, full scores (64). This means every race, you have to win first place so you get 16 points each first place victory. You’ll get two spoilers every completed cup, it’s intended.
Get all the spoilers. Complete all cups on Grand Prix in any cc. And first place.
Complete all Grand Prix cups in 150cc, in first place for all races like usual. You can get this and Spoiler-flower by doing all cups in 150cc, you don’t need to do it individually for each cc.
Win your first puzzle piece. If you play solo (not multiplayer), you’ll see three puzzle pieces around each race track. Just drive over one, and you’ll get it. Some require items, some just need you to drive through certain hidden places.
Complete an image. Collect puzzle pieces, and eventually they will form one. 3 pieces per image. Remember that all puzzle pieces on a track aren’t for the same image.
Get all 48 puzzle pieces. This is the longest-winded achievement likely. You need to collect each piece, some of which require items not on time trials so you’ll need to play through Grand Prix for some. I found a good video on all the locations. Video not made by me, timestamps in the original video’s description.