Who hasn't? I used to get so frustrated when my cakes would turn out uneven. I cursed my oven, endlessly researched bake even strips (because of course a gadget will all my problems). But then, I realized something...frosting covers a multitude of sins...well frosting and a little clever repositioning of the cake.
First off, if you have a dome in the middle of your cake, it's going to have to come off. This is by far the most challenging bit. It just takes practice to be able to cut off the dome and keep everything flat. I am actually terrible at it. I will fix the problem with some frosting later on. Anyway, I digress...
I find it easiest to do this when my cakes are cold. I like to wrap them in plastic and refrigerate or freeze overnight. Frozen cake is still cuttable. Fresh cake is often a crumbly mess. Even if you are running short on time, whip 'em in the freezer for a few minutes.
Typically my a top edge. I lay my bread knife (or the longest serrated knife you have) parallel agains that top edge and then gently saw off the top keeping your knife even. Lift off the dome.
You may have to turn the cake a few times and take a couple passes with the knife to make sure you have gotten the cake even all the way across.
Next, we add some frosting. For the middle layer of frosting, especially if you are using the same frosting as the outside of your cake, it is not super important that it is perfectly even, but you want it pretty even because your cake sitting on top of that frosting will lean if you have too much gooped up on one side.
You want to repeat the above steps to even up your next layer before you add it on top. But then, here is the trick, put the cut side down on top of the frosting. You already know the bottom of your pan is totally flat and it will give you a smooth flat surface to frost.
Lastly, frost the tops and sides. I find it best when frosting the top to get down on the same level as the cake to make sure everything is even and I don't need a little more frosting here or there ( kind of like when you pour into a measuring cup). I actually find it easiest to put my turntable and cake up on a cake plate to work on it instead of hunching over the counter.