Recently, I've gotten a lot of questions about cakes, so here is a 101 of sorts for your convenience.
If you are doing a big cake, whether a sheet or 100+ servings, try the recipe out first. Cut the recipe in half and make a few cupcakes. Make sure that the cake has a good crumb, no tunnels, good moisture content, flavor, and gauge how delicate it is.
When mixing the batter, typically you mix the wets, including sugar, and the dries separately. You then alternate and mix just until combined. You don't want to overbeat the batter, or it will be stiff. When using dairy products, such as butter and eggs, bring those things to room temperature. Other items, such as milk, sour cream, etc. should be set out for a bit but not too long or it will spoil. When mixing hot liquids with eggs, you want to temper the eggs first. Add a bit of the hot liquid to the eggs and mix. Add a bit more and mix. If you just dump them together, you'll have scrambled eggs in your cake. Not too yummy.
If you don't have cake flour, or don't want to buy the expensive stuff at the grocery, you can make a substitution. Measure out the flour that is called for, using all-purpose flour. For every cup, remove 2 Tbsp of the flour and add 2 Tbsp cornstarch. Sift and sift and sift and sift again. Seriously. Sift 4 times so that the cornstarch is distributed evenly. You can, of course, use regular all-purpose flour, however the texture will be more dense.
To prep your pans, use parchment whenever you can, and grease and flour. You can use butter, shortening, oil, but I always go with a nonstick spray. Toss in a couple tablespoons of flour, or cocoa if making a chocolate cake or brownies, and tilt the pan to evenly coat. Knock any excess out into the trash.
After baking, let cakes cool in the pan. Here there is a little variance. I prefer to freeze in the pan; others depan, sometimes wrap the cake, and freeze. Freezing in the pan ensures the cake freezes in the correct shape, and depanning is much easier. Usually, I can just bang it upside down, and the cake comes right out. Then you can brush on a simple syrup and toss back in the freezer. The syrup adds flavor, moisture, and stability. Once the cake is frozen it is very easy to handle and sturdy. I also prefer not to wrap, instead leaving the layers open on a baking pan lined with parchment.
Level off your layers, unless you really don't care. I only do this when it is for someone else or for work. More cake, more better for me. If leveling, use a bread knife or other serrated knife. I cut outwards in, while rotating the cake on the counter. (Use parchment or a board!)
To keep crumbs at bay, use a crumb coat. This is a very thin layer of icing that acts as a primer. You can a portion of the icing to thin it. You want the crumb coat to become one with the cake. It doesn't sit on top but fills in all the nooks and crannies. Freeze again. Trust me, it helps.
When icing, always use room temperature frosting. Sometimes beating it on high for a minute will help keep a smooth consistency. There are several different methods to ice a cake. I typically pile about 2 cups (for a 9" layer cake) on top and spread it out in a circular pattern, overlapping the sides. Then I either hold the cake with my left hand or use a pedestal stand and rotate the cake while icing the sides, again, overlapping the top. A dough cutter works great with this, since it is usually as long as the cake is tall. With an offset spatula, rake the edges inward, keeping it level. Once you're done, pop it back in the fridge.
If you want to try something fun, just swirl the icing around with a knife, or flick it out with the tines of a fork. You can also use the tines to make interesting patterns. It doesn't have to be all smooth and pristine. Cakes are fun!
If you are stacking tiers, stick a dowel, straw, or skewer into the bottom tier. Mark the depth and cut 4 more the same length. Arrange in a star and push all the way to the bottom. Add the next layer, making sure to stay centered. I've only done 3 tiers, so I can't give much advice on super heavy and big cakes. However, this is the method I use.
When cutting cakes, or really anything, I use a sharp knife that has been dipped in hot water. Sometimes I dry it, depending on what exactly if being cut.