As I have wrote who Forseti was in a previous post, I will tell you know who to wor with them in a spiritual way. Fortunately he is one of the Norse deities that people are able to work with and that kind of knowledge is passed down to us.
Forseti is the god of Justice of the Norse pantheon. He is the god to turn to when people are in need of justice, any kind of justice. The problem has to be exposed to Forseti and he will judge if it is a worthy one. Take in mind that he will be at the side of those who have the reason, of those who are righteous and honorable, so before you call upon him, you have to be absolutely sure that you are being wronged and that the law, truth, honor and good will is on your side. Also, you have to make sure that your own problem isn’t yours to make amends for, if you aren’t able to fix your own problem by yourself, your own strength and the power within you, then you may call upon Forseti.
He gives valour to those who are able to defend themselves with reason and speach, you can ask his help on that field, and not to do something for you, unless as I have mention it, if you aren’t able to do so because of reasons higher than yourself.
Forseti is also the god of fairness and equal exchange, equal rights, so you have to be sure that you are clean in the problem that you are facing.