The mechanics of describing and appraising it would not be significantly different than any other appraisal. If you could find a similar shaped lot to use as a comparable, that would be ideal, but its not likely and its not an insurmountable problem.
Presumably you're asking because the shape is awkward. In that case the appraiser will look at the highest and best use of the property. If, for instance, we were talking about grazing land, livestock could graze right up into the corner, so there wouldn't seem to be much basis for a seeing the shape as unfavorable; a cow doesn't care what shape the pasture is.
If you're talking about a residential lot, set-backs may make the corners undevelopable, and the total area of the unusable corners may represent more yard space than the public typically demands (or will pay for). In that case you'd figure out how big a building foot print the lot can take, add a reasonable amount of yard space and derive price from similarly sized lots (similar utility). The excess corner space would just not add any value.
The thinking would be similar for a commercial lot.
Describing and accounting for a lot with usability problems is not a problem for a competent appraiser. I do this frequently, but not usually for lot shape, but instead lot slope. I work in the mountains and you frequently have a multi-acre parcels that have less than an acre that are usable because they're on the side of the hill. The value is less than if the lot was flat because it only provides set back from neighbors, not set back AND space for animals, a barn, a shop, etc. IOW it has utility similar to a smaller parcel. That's what you appraise to, the value in use.
If the area consumed by set backs is so great that the usable portion of the lot is no longer usable, you have to consider a different use or the possibility of getting a variance. In that case an appraiser might give an "as-is" value based on some limited use. He might say that, without a variance this lot is only usable as additional parking lot for the building on the next lot over or as additional yard space for the next house over. Alternately he might give an opinion of market value based on a "hypothetical condition" (a condition known to be contrary to what actually exists). In that case he might say, "the lot is worth $x based on the hypothetical condition that a variance can be obtained that will allow the buildable area of at least xx square feet."
Making an adjustment to the size based on usability considerations is something most appraisers should be able to handle. Dealing with a substantially atypical best use or a hypothetical condition can be a little more tricky.