I just finished course 1 of Wilton cake classes, and I was asked to do a simple wedding cake. I was wondering how to do a two or three tier cake. Do you layer them on one another or do you put the same size plastic under each layer? Please help me. Thank you 
Here is alof of information I have gotten from here: It's alot but it helped me:
Stacked Tier Tips
A lot depends on the size and weight of that top tier. I don't use the flimsy white cardboards, the greaseproof ones that Wilton and other make; I don't find that they are strong enough. I do use the foil covered cardboard ones. For a really heavy top tier that is large, I would double up having the lines of the corrugation going in different directions for added strength but normally one will do. I know some people do use the white ones or the white and silver ones but I don't find them strong enough at all. What I often do is either glue gun two foil covered circles with the foil sides facing outwards and glue gun a circle of parchment to it, or use one and glue gun a circle of parchment paper to the bottom.
You can use the plastic separator plates and a circle of parchment under it, to stop any sticking to the lower cake. But if you are going to transport the cake and need a centre dowel you cannot do this.
I nearly 100% of the time, use a centre dowel for any stacked cake. Sometimes I do take a chance and transport a two tiered cake without it though, but if you have to brake suddenly, the cake has nothing holding it in place except the icing and can shift inside the box. It hasn't happened to me yet but I know many that have had that experience.
Parchment is a great idea too! Do you let the icing crust before you place the parchment?
Yes, I let it crust first and still use a bit of powdered sugar or cocoa for chocolate cakes. This is my new favorite method, haha! The only drawback with separators or using Masonite and such is that you cannot insert a centre dowel if traveling with the cake stacked.
No, I haven't ever had a cake shift but you really have to level those cakes and have the dowels slightly lower or level with the height of the icing.
This past Monday my Wilton instructor said that whenever you stack cakes you should put the support dowels to hold up the board and one long one through the center of both cakes.
Fast and Easy Push-in Leg Construction
Dowel rods are not needed, because legs attached to separator plates push right through the tiers down to the plate below.
Ice cakes on cake circles. To mark where legs will go, simply center separator plate for tier above (projections down) and gently press onto the tier. Lift plate off. Repeat this procedure for each tier (except top). Position upper tiers on separator plates. Decorate cakes.
To assemble: Insert legs into cake at marks. Push straight down until legs touch cake board. Add plate with cake to legs. Be sure plates are securely fastened to legs. Continue adding tiers in this way until cake is assembled.
Hints for Tiered Cakes
Before placing separator plate or cake circle atop another tier, sprinkle a little confectioners sugar or coconut flakes to prevent plate or circle from sticking. Letting icing crust a bit before positioning plate on cake will also prevent sticking.
You will have less crumbs when icing if cakes are baked a day in advance.
When filling or torting large layers, use less than you usually would. Your dam of icing should also be far enough from edge so filling doesnt form a bubble.
The cake Icer tip (p. 789 - from www.wilton.com) is an invaluable timesaver in icing wedding tiers.
When transporting tiers, place cakes on non-skid mat, damp towels or carpet foam and drive carefully.
To keep balance, cut cakes on the Tall Tier Stand from top tier down.
To divide tiers, use the Cake Dividing Set (p. 122 - from
www.wilton.com). The Wheel Chart makes it easy to mark 2 in. intervals on 6 to 18 in. diameter cakes. The garland marker gives precise spacing for stringwork and garlands. The raised lines on separator plates can also be followed for each dividing.
When using Spiked Pillars and stacked construction, double cake boards or use separator plates between layers to prevent the weight of tiers from causing the pillars to pierce through cake.
I've found that dowels are a lot cheaper and just as effective as the push pillars. If the push pillars are something you don't already have, don't go buy them just for this cake. Use the wooden dowels.
I think your sizes will be fine stacked. I would make sure to put enough dowels in the bottom tier to make sure to support the weight of the other cakes. I just make and decorate the cake, and then cut the dowels to the exact height of the cake and then place them in the cake in a circular pattern at least 1 inch apart. If you are putting a 10" on top of the other 10" I would make sure to at least make a 6 ot 7 inch circle out of dowels. Then for the second 10 inch cake I would make a 4 or 5 inch circle out of dowels since you are putting an 8 inch on top. Hope my directions are clear enough.
As for cutting the cake, Some people cut the cake still assembled, others, like myself, pull the cake apart first. I do this since I often make my layers different flavors. I have to take the top flavor off to get to the middle flavor if someone wants that one. It really is your preference. Make sure not to frost over your dowels since you will need to see where they are to pull them out before cutting the cake. Always use cardboard cake circles or cake plates between the layers too. Never stack cake on cake. It will not hold and be way too messy to serve if it should for some reason hold. Hope this helps. If my instructions are very vague, you can pm me. Always glad to help!
I wanted to add a trick I recently learned from another post -
Before stacking your tiers, leave the dowel exposed a bit so you have finger room to place the next tier. The weight of the cake will push the dowels back into place. Some people said they pushed the dowels in
first to ensure they were the appropriate length and then pulled them back up again. Others said they just measured the dowels against the side of the cake, and then put them in leaving them exposed. Get your fingers out of the way quick though.
I just made my first two stacked cakes. I'm getting better at the assembly. Taking them apart is another story.
I Forgot to add - If you have a Wilton yearbook, they have a good guide for cutting the different size tiers.
I just thought of something...are you making a tiered cake or a stacked cake? A stacked cake is where it looks like the cakes are just sitting on top of each other and you can't see between the layers. The tiered cake is where the cakes are separated by several inches and are often placed on the push pillars or columns.
If a stacked cake.. Keep to what I said before, if a tiered cake, this changes a lot of things from my other post. You would definitely go with the push pillars.
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