This is not easy and is usually done more in swordtail's to cross between different species.
What they do with swords and gently rub the tummy of the male, placing you finer on the central tummy area and then push gently down towards the gonopodium, if done correctly the male produces a small amount of milt (visible with swords but not sure with guppies).
Then if u get the milt extract it and then place it on a female vent area, in normal mating males do not insert the milt into the female but only places it on the generatal opening. The female then absorbs in the milt to fertilise the eggs.
I don't know if it will work well with guppies but let us know what u got.