easy

you take a piece of string, and draw it through one of the holes that the fan should have in each corner. stick it through all the holes, in a way so that you get the string to lie on top in two of the sides, and below in the other two sides.
now, losen the string a bit so that you can take the string, in the two sides where it's "below" the fan (ie: you've put the string in from the top and it's coming out below, through the hole) and put it under the edges of the heatsink on your voodoo3 (hope you've got v3 3k, the heatsink sticks out to the sides). it fits nicely (my celeron fan does) and now you just take the two loose ends of the string and tie them tightly together.
this might be confusing, but for good reasons I can't scan a picture of my video card to show you.....