Mostly it comes down to practice.
However there are some tips and tools that can make it a lot easier.
If you have a cake stand (the kind that spins) then you set the cake on the stand and slowly turn it with one hand while cutting forward and back with the other (trying to keep the cutting hand as level as possible).
Another option is to buy a cake leveler http://m.wilton.com/store/site/product.cfm?sku=415-815 - if you make a lot of cakes it can be very handy.
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I had always heard you could level a cake by taking a large length of dental floss, surround the cake at the level you need, pull tight till the floss goes all the way through.
Then if it's not quite right, flip that sucker upside down (the bottom is completely level after all!).
In the future, if it is just out of the oven, you can push down on the cake using your flat hand and a clean towel to help push any "dome" down.
And, for the future, you might want to try baker's strips to get an even cake from the get-go.
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Here's the trick my mom taught me.
If your cakes are tall enough that the entirety of the "domes" are higher than the sides of your pan, return the cakes to the pans after they've cooled completely and use the edge of your pan as a guide for trimming the domes using a long knife.
This trick tends to work pretty well for me, however sometimes I need to make a double batch of batter to get my cakes tall enough, depending on the pan.
I personally don't mind making the double batch since I use any excess batter for cupcakes and the cake scraps are my favorite part of the cake, but it may not be for everyone.
Hope this helps!
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Alton Brown on the subject (from "The icing man commeth
" Season 6 episode 15).
...
in case the link goes down:
His solution is to use a bow saw blade (or a long serrated knife) to cut and 2 yard pickets (or scrap pieces of wood) as a guide.
For what you would spend on all that, you can just get the cake leveler, but I do like the idea of using a guide of some sort.
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Use toothpicks and a ruler to mark level points all around the cake by pushing the toothpicks about half the way in at the height you want.
Then use a serrated knife and cut from toothpick to toothpick.
I cut my cakes into 4 1 inch cakes per stack to put filling between and ended up with 3 5" stacks total and they were all perfectly level.
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Turn it over, the bottom is level and once you turn it over the dome, now the bottom, will settle evenly.
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If you want to eyeball it vs. use guides or measure, your best bet is to cut only about halfway across the cake.
Then turn the cake a 1/4 turn and cut halfway again.
Turn it 1/4 turn and cut it again.
3 times normally does the trick.
By now the top will release.
If it's not loose, turn and cut again.
This method keeps you from dragging which creates rippled, uneven cuts.
Also, it is easy to eyeball where to start cutting.
It's the last half (if you cut all the way through) that you typically go off course.
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Make sure the cake has cooled.
If the cake is frozen, let is thaw slightly.
Trim the uneven cake layer with a long serrated knife so it is even.
To slice one cake into two layers, start by tracing a line around the middle of the cake with a long serrated knife.
Then slowly rotate the cake while following that line with the knife and cut through the cake toward the center.
After a few rotations you will have sliced the cake in two.
Repeat the process with the additional cake layers.
To separate layers, 8-inches and above, carefully slide the top torted layer onto a cardboard cake round.
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Use DIY cake strips (or you can buy them) to prevent the dome.
A little preparation = less effort + better result...
I think?
DIY cake strips
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If you cake is too domed, you can just cut the top off, right?  Thing is, I tried this, and now my cake is taller on one side than the other. So it's still not flat! ;-)  Does anybody have any good tips for keeping the cut level?  (I already know you need a knife long enough to go right through the cake. I have that. It's just difficult to cut in a straight line by hand.)
How to trim a cake so it's level?
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cooking.stackexchange.com/users/10926/MathematicalOrchid
Use a spinning cake stand while cutting at the same level, or trim the uneven cake later with a long serrated knife. In future, cakes can be levelled by surrounding with dental floss at the right height before pulling tight, pushing toothpicks in at the right height around the cake or using a cake leveller. Spin the cake on a cake stand and slice with a long serrated knife. You can also buy a long serrated knife.
Spin the cake on a cake stand. Try using a long serrated knife. You can buy a cake leveler or cake strips. You can use dental floss or toothpicks and a ruler to mark level points.