I've never spackled before and I tried to fill in some space between the wall and the stairs after we took out our carpet. Suffice it to say I used way too much spackle and now I have an uneven mess. What is the best way to fix things now that the spackle has hardened?
Get yourself a drywall sander
Sand down the high spots using rough grit (100 grit) sand paper (they sell it to fit the sander - you'll find the sander in the drywall area and the sandpaper in paints) then smooth it with 200 grit.
If you have low points, fill them in after sanding.
Next time use less mud (little applications, not big ones) and then use a really really big sponge that's been slightly dampened to even it out.
When there's a lot of stuff to knock down, I like sanding screens on the kind of drywall sander that attaches to a dust collector bucket (you can get inexpensive ones at big box home improvement stores) which is in turn connected to the shop vac.
Saves a ton of effort and cleanup.