It takes some practice getting used to the way Falco spikes. A great way to get used to Falco's sweetspot is to go into Wifi mode, start a new game and practice spiking against the punching bag. The goal is to spike him as many times in a row as possible. Go. Keep a rythm. Eventually, dash towards the punching bag and land a moving spike. Spike in several different situations. After you feel like you've mastered the exact timing, U-smash (or F-Smash) the Punching Bag off the ledge and spike him to his doom. Repeat until you feel confident spiking in a game.
I think one of the most useful things to practice is chain-spiking. Shot hop, DI right/left, spike, short hop, DI right/left spike and repeat. Although it's very situational, it can still work as a great approach. But really, the only way you can get 'good' at spiking is by practicing. Use it whenever you can. Try spiking level 5-7 CPUs. If you're still having trouble, fight a Lucas and knock him off the ledge. When he uses his Up-B recovery, he's screaming "Spike me!"
Eventually, practice in real matches.