It hasn't been mentioned, but you will want to turn magnetic snap on, if it isn't already (it's the icon in the toolbar that looks like a magnet). If you select the rectangle while it's still a Rectangle (before you Arrange->Convert to Editable) and enter the Rectangle tool, you'll see a little x in the center of the rectangle that you can snap your guidelines to.

Also, I apologize as I was going to mention that you probably would want to use a couple of guidelines, but I completely forgot to mention it.

I only know one way to do mirror bending so that the sides bend at the same time, but it only works on squares and all sides move, not just the ones you want. However, in a pinch you can draw only half of the rectangle, then make a copy and mirror it and join the two together.

Don't get too frustrated, though! One of the neat things about Xara is the many different ways to approach pretty much everything that you want to do. In the beginning it can be taxing trying to figure out how to do things, but once you start getting the hang of what is possible, you're suddenly able to apply knowledge from previous problems to new ones.