I'm looking for an easy, simple way on how to make fondant stripes for a double layered 8" round cake covered in buttercream. It will have a 6" round on top, it was a last minute cake I'll be doing for a friend but I've never done fondant stripes before. I dont even know how thick the stripes should be! Any suggestions or tutorials I can take a look at? This is the cake she liked but with a few changes. 
What I usually do is measure out the stripes according to the proportion of the cake. Like if the stripes were on a 6 inch cake they would be 1/2 inch thich or so but if they were on a 10 inch they would be around an inch thick. It really depends on the look you want to go for. If you want there to be more stripes on the cake then the stripes should be thinner. If you want a thicker look with the stripes then they should be thicker. I just measure out the stripes with a clear ruler and cut them with a pizza cutter then add to the cake with a little bit of buttercream. Hope this helped!
Roll your fondant out about a 1/8th of an inch thick. You don't want it to be see through, but I wouldn't go super thick either.
There are strip cutters out there, but you can use anything with a straight edge and a pizza cutter. I'll often use a ruler. Your strips can be the same width or you can mix them up and do different widths.
I would use a cake plate to mark the top of your cake where the 6in will sit. I wouldn't worry about cutting all the strips the exact same lenght. I'd get a basic lenght, lay the over the sides. You can use a little bit of water or vanilla extract to glue it down. I'd then take a knife of the pizza cutter to trim the fondant at the bottom edge.
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