Hi Wendy, you say you got a jig to cut dovetails. Do you mean you have a jig to make dovetails with an electric router ? Deanbecker took that to be your meaning.
They are sold by companies like Leigh. I had a cheap one years ago and it worked okay, and was even fun for a while. If it is a jig for an electric router, it should come with directions. There are probably also youtube videos for whatever jig you bought.
Or did you get a small magnetic jig to guide a saw for hand cut dovetails? Folk on Paul Sellars’ website are interested in hand cut dovetails, not electric router cut dovetails. Paul teaches us to cut them without any jigs, except a marking tool for laying out the dovetails at set angles. But I know a number of places sell jigs, usually magnetic, to help guide saws to keep them straight in the cut. For example, Veritas (Lee Valley) sells one. Amazon also has some.
If you are interested in handcut dovetails, this is the website for you! Paul has detailed videos for how to cut them, along with other basic woodworking joints. I think they are found on his Common Woodworking site but I first saw them on youtube — those free youtube videos were my first introduction to Paul. Again, he does not teach us to use a magnetic guide, but there is no reason you cannot use one. However, I would suggest first practicing a bit without a guide till you get a feel for how dovetails work — some folk, and that means me, can have trouble visualizing how they fit together at first. Once you get that down, you can use the guide to cut straight, though you may soon find it more fun to try to cut straight without a guide.
In any event, whether you use an electric router to cut your dovetails, or try it by hand, practice first or scrap. You do not want to mess up your project. An electric router guide system can be complex and you can screw up. Hand cutting, even with a magnetic jig, requires some skill. Handcutting with out a jig can take some time to get decent at.