They are actually very easy to make and you can make a liter for roughly the cost of a small bottle of Swedish bitters.
First choose what herbs you wish to use. Some good choices are gentian, mugwort, yarrow, artichoke, horehound, dandelion leaf, milk thistle herb (not seed), or picrorrhiza. Artichoke leaf, milk thistle herb, and picrorrhiza are also very supportive to the liver.
Partially fill a glass jar with the herbs you wish to use. Add some alcohol (vodka or gin work well)to cover the herbs by several inches. Put a piece of wax paper over the mouth of the jar if the lid is metal to keep the alcohol from coming in to contact with the metal. Then put the lid on tight and shake it up. Shake it up several times a day for 2 weeks or more, then strain the alcohol from the herbs and store the liquid in glass bottles. That's it, you have your own homemade bitters.