Fondant is a thick sugar-based icing that can be moulded into shapes and figures. Fondant is often used on cakes because it provides a smooth covering that can be used as base for the other cake decorations. If you want to make a striped cake, you first have to cover the entire cake in fondant because the stripes need something on which to adhere.
Cover your cake in a very thin coating of buttercream. Set the cake aside in a cool place to dry.
Put the cornflour and icing sugar in a sifter and cover your work area with the mixture.
Place the fondant that will form your base coating on your work area. This fondant can be white, although you may use any colour you wish.
Roll out the fondant using a rolling pin and just a slight amount of pressure. If the fondant is too hard you can knead in a small amount of shortening, and if the fondant is too soft you can work in some icing sugar.
Lift and turn the fondant after every few passes with your rolling pin to ensure it doesn't stick to the work area. Continue to thin out the fondant until it is about 6 mm (1/4 inch) thick.
Roll the fondant over the rolling pin to move it without breaking it. Take the fondant over to your cake.
Place the end of the fondant that is hanging off of your rolling pin onto the table directly in front of the cake. Move the pin over the cake, letting the fondant roll off of the pin and onto the cake as you go.
Place the flat surface of the fondant smoother on the top of the cake. Move the smoother in a circular pattern around the top of the cake, using light pressure, until the top of the cake is perfectly smooth.
Lift the fondant on the sides of the cake slightly while you press it into place. Do this all the way around the cake until you have removed all of the wrinkles from the fondant. Cut away the excess fondant that is left at the bottom of the cake.
Roll out another stretch of fondant at your work area, using the exact same process as before. This time use the fondant that is the colour of the stripes that you want to add. Make this layer of fondant 6 mm (1/4 inch) thick as well.
Place your straight edge over the fondant and use it as a guide to cut your stripes with a knife. Move the stripes from the work area to your cake.
Apply a small amount of water to the underside of the stripe and move the moisture around with your finger so the whole side is damp.
Press the fondant stripe into place on the cake and hold it there for several seconds. The wet fondant stripe will bond with the fondant cake coating. After a few seconds, let the stripe go.
Continue to cut and apply the stripes until the cake looks the way you like it.