agreeing with weefoal's advice, and after the groundwork and commands, and getting her proofed in traffic etc., by which time she will be a bit older, you then can also get her used to things gradually and in stages as she gets confident with each stage, so for instance, later when she is long reining happily, you can start to get her used to the feel of something behind her. A good way of doing this is to have an assistant with you, and, having attatched the traces, and while you are handling the reins, have your assistant pulling back on the traces, as they walk with you, just a gradual pressure, so that she begins to get used to the feel of something against her breastcollar. Later you can get the same person to drag something behind, just to get her used to noise from behind, and when this is all going well, and she is happy, you can attatch a trye or a log, for her to pull; get the assistant to take the weight of the object at first, and gradually let pony feel the extra weight. I can't stress enough the need to take your time between each stage, before moving on to the next, so that you have a happy, safe pony that is a joy to drive. When she is happy doing this for a couple of weeks, you can add something like broom handles put through the tugs, andtied with balertwine to the breeching straps. This will get her used to the feel of something coming into contact with her sides as she turns; have a person at her head to help her when she first tries turning left or right , and make sure they are not so long as to be near the reins or her head in anyway. And then after all of that, I would think about the cart. Its best always to have an experienced and respected driver /trainer with you and always have an assistant to help you, if you can't aleways have a trainer. All of the time and money will pay off, and its fun, so well worth the extra effort of not taking shortcuts.