What is the most important thing if I substitute yarns?
You will have to do a tension. The tension is always done in stocking stitch unless specified.
My yarn is heavier then recommended in the pattern. Can I use smaller needles to get the same tension?
Unless you make an outdoor jacket I do not recommend it. The fabric will get stiff. It is best to stay with the tension recommended on the label of the yarn.
If I use a worsted yarn instead a double knit can I use the same pattern?
If the tension only varies ½ stitch per inch you probably can use the pattern by going up or down a size or two in your pattern.
My husband picked up a pattern for double knit but I use worsted yarn, how can I adjust the pattern?
Figure out how many stitches you would have to use with the tension recommended for your worsted yarn to make the proper size sweater. For example: Finished size 42 inches. On your label it says 20 stitches to 4 inches or 5 stitches per inch. That means you would need 210 stitches for a sweater in the round or 105 sts for front and back. Now check in your pattern to find a size that uses close to 105 stitches. Follow this size for width and all the shaping. For the length follow the actual size.
If I don't find a pattern I like in the correct weight of my yarn what should I look for?
If you use a heavier yarn then what is used in the pattern make sure the size you want to make is one of the larger sizes. The opposite works for smaller yarn then you want to make sure it is one of the smallest sizes.