How can I make a balance scale ,<850 pound capacity?

If you search google for "homemade balance scale" you will find lots.

I can't recommend any until you decide on a capacity. < 850 lb is pretty vague, a scale that had a capacity of 1 pound would meet that criteria.

edit. 750 lb? yes, you can do it, but you will need a large structure. A 4x4 beam should hold the weight. If you make the short arm 1/10 th the long arm, eg, 1 ft and 10 ft, you only need weights up to 75 pounds, and down to 0.1 pound. Weights of 0.1,0.2,0.4,0.3, 1,2,3,4, 10,20,40 would do.

The difficulty will be designing it so that the pivot has low friction but is still able to hold the weight.

.

You would have to leverage like all balance scales.

If you use a single beam, that beam would have to be able to support the load without shearing or bending. It would also be HUGE.

Alternately you can use gears which would make it more compact but much more complicated to build.