[begbrook]

I have bought some prepared hyacinth which I want to grow in water in bulb vases. How do I force them? Do I put them in compost/sand as if they were going to be planted in compost?

[snowdrops]

I think you just fill the bulb vases with water, pop the bulb on top & it will start to put out roots.

[begbrook]

I know they have to be kept in the dark for 8-10 weeks to force the bulbs to sprout,  I did this last year in sand then potted them in compost to grow indoors.  This year I want to grow them indoors in water so do I force them in water or sand/compost.

[New shoot]

If they are pre-prepared bulbs and you want to grow in water, you just sit the bulb on top of the vase as snowdrops said. 

The forcing them in the dark bit is only for bulbs you want to grow in compost 

[SusieB]

I grow hyacinths in bulb vases.  I put them in a cool dark place for a couple of months (or when I remember they're there) whilst they develop roots and a short shoot.  And keep the water just below the bottom of the bulb, i.e. not touching the water.