Check out the Hippeastrum/Amaryllis Forum on this site. You'll find lots of info there. If your bulb hasn't rotted, it should be o.k. Cut off dead roots and keep the healthy white roots. The bulb should be planted in a coarse soil mix with very good drainage and make sure at least 1/2 of the bulb is above the soil. Water sparingly at first.

Places sell the kits with a vase and stones to grow amaryllis is water, but it is not a very good way to grow amaryllis.

Yes, your amaryllis was starting to rot. The white roots are the healthy ones you want to keep; the others will probably just compost into the new dirt you have them in. It should be fine as that has happened to me before as well. I have never lost any.