What's the correct way to wield a Longsword?

I've been practicing with Katanas for many years, and I've never had any problems cutting with them. However, I've recently purchased a European style Longsword, and I tested it on some water bottles, and it only baseball batted them across the yard. It's a functional carbon steel longsword, and I sharpened it myself and I'm pretty sure it's sufficiently sharpened. I sort of have the Katana style stuck in my head so I'm probably doing something wrong with my form. Please help if you can.