Jamie Wallen (The Apothecary's Apprentice) has a great video about tension. Handiquilter has one too as does Helen Godden. You can find them on You Tube. Start with a sample sandwich with the same fabric and batting. Just a strip will do. Adjust your bobbin tension first using a "fall test". The videos show you how. Then adjust the top tension as needed. Remember that on long arms you need to turn the tension dial at least a half turn to see any difference at all. It isn't like a domestic when a tiny turn makes a difference.