Because they're so close to the ground, even standard-size dachshunds can benefit from some sort of warm, protective clothing when they have to go outside in the winter. This is especially true if there is snow on the ground for months at a time. That doesn't mean they can't also wear something cute the rest of the year -- maybe a hot dog costume or a pair of mock doggy lederhosen to reflect their German heritage.
Measure your dog. Before you can begin to design any clothing for your dachshund, you will need some basic measurements: around the bottom of the rib cage (where the "waistband" would go); halfway from the bottom of the rib cage to the tail (anything further down the belly may be peed on); the bottom of the stomach to the ground; from the bottom of the ribcage to the base of the neck; and from the bottom of the rib cage to the centre of the chest. These measurements will help you design clothing that fits your dachshund perfectly.
Make patterns. With your dog's measurements in mind, construct basic pattern pieces out of cheap muslin (so they can be used more than once): shirt pattern (T-shirts, hoodies, short jackets); skirt pattern (not recommended for heftier dachshunds as the skirt may drag on the ground); and coat pattern (longer in the back than the shirt pattern). Baste pattern pieces together and try them on your dachshund to make sure they fit, then remove the basting and make the outfits for real.
Consider how to fasten the clothing. Use Velcro to close all waistbands and shirt fronts for "easy on, easy off."
Be sure to wash and dry all fabric before cutting and sewing to make sure any shrinkage will already occur. Remeasure your dog from time to time, particularly if he is less than a year old (he could outgrow his old measurements) or has recently gained or lost noticeable weight. Because of their tendency toward back problems, consider making some warm clothing for your dachshund for the winter months. A warm back is likely to be a healthier back.
Because your dachshund is so short, be careful when you're making her a skirt. Too long a skirt can trail on the ground and be stepped on by you and your dog.