Manual flash can be tough. You've got to use your distance settings to balance the light, which restricts you.
Basically, if the sunlight setting calls for f/22 at 60th of a second (roughly right for 100 speed film) then you want to hold the flash as such a distance that its scales say somewhere between f/11 and f/16 worth of light are hitting the subject. The ideal for fill is one and a half stop below the ambient highlights. Makes it look quite natural. Putting the flash on a longish cord can help quite a lot, because you can then adjust the distance by how far away you hold the flash.