Add water to base of bulbs and maintain water at this level. Don't submerge bulbs; they may rot. Keep water topped up, and when roots have grown to about 2.5 cm (an inch), keep water 6 mm (1/2-inch) below base of bulbs. Place bulbs in a cool (5° to 10°C or 45 to 50°F) dark place for about two weeks. Warmer temperatures stimulate stem growth at the expense of root growth, and heavy flowers will topple the plant over. Strong roots appear within a matter of days. When shoots appear, move to a cool, sunny spot, 15 to 20°C (60 to 68°F).
After blooming begins, you can prolong the blooming period by placing the container in the refrigerator overnight, or in a chilly room. Discard the bulbs after blooming. Forced bulbs can seldom be forced successfully again; they've expended too much energy to recover.
Related web sites
The American Daffodil Society
This site is jam-packed with information about all kinds of daffodils. Great photos showing representatives in each of the 13 daffodil divisions. (Paperwhites are in Division 8.)
The Chilling Facts About Forcing Flower Bulbs: this site gives good instructions on how to force other spring-flowering bulbs.
This list from The Netherlands Flower Bulb Information Center gives names of bulbs that can be forced, and how many weeks they need to be chilled before they bloom.
Online sources for 'Ziva' and 'Grand Soleil d'Or'