To echo the others above, I've seen two basic types:

Four load cells, placed under the fore and aft of each hull, connected back to a central display.

One load cell, hanging from a winch on a frame (an old steel swing set works well).

As mentioned, weigh the boat with the mast down. Be sure to include all boat equipment (sails, blocks, tiller), empty the hulls of water or other gear, and remove all removable gear (tramp bags, etc.).

Putting this weighing gear together is somewhat of a PITA, so I would not suggest bothering unless you have reason to suspect the weight is WAY off, and you intend to race at a major regatta where the boat will be weighed, and you need to make corrections in advance. Otherwise, just sail it and get it weighed at a major regatta when the time comes.

Hope this helps.

Mike