Definately cover your whole cake with fondant first- Hope you have luck with the Wilton cutter - myself I would prefer using a pizza wheel cutter ( gives a nice sharp clean cut with no dragging)
Pop into an art supply shop or even Office works - grab a few sheets of Frisket - it is like really low tack clear 'acetate' looking sheets. Measure your cake and calculate the strips (width, height, number required and colour placement) Make sure you do white strips too - don't just pop on the blue strips and leave the base fondant for the white parts.
Lay your cut fondant strips on to the Frisket, using a metal straight edge to keep them straight - you'll have to cover the strips with gladwrap so that the don't dry out whilst you are working - try and work as quickly as you can. Give the surface of your cake a brush with water - wipe off any excess with a paper towel.
Pick up the whole sheet of the Frisket Sheet and wrap around the moistened cake - fondant strips to the inside - Frisket sheet on the outside. give a quick rub over to ensure strips are secured to the cake.
Carefully peel off the Frisket sheet....... you now have perfectly straight, aligned stripes - no stretching - no distorting and NO crooked strips :)