To be called a fricative a sound segment must be articulated with such a narrow stricture (or close approximation or sufficiently small aperture of the articulators) as to cause turbulent airflow during the medial stage of its production. The auditory result of this turbulence is friction. So for a sound segment to become a fricative certain auditory/articulatory/aerodynamic conditions must be fulfilled. I need not go into most of these details. This has most competently been done by J.C. Catford in his book Fundamental Problems in Phonetics (Edinburgh) of 1977.
German speakers who produce an English /v/ which is too similar to the /v/ in German words like Waldi, Wanne, Wasser, wo, wann should press the lower lip much harder against the upper teeth and exaggerate the resulting friction. Don't worry if you overdo it. It'll wear off.
There was an Old Man of the West,
Who wore a pale plum-coloured vest;
When they said, 'Does it fit?'
He replied, 'Not a bit!'
That uneasy Old Man of the West.