Good blending isn't just about the cut.
Its also about getting the right colour, correct ventilation, hair type & density AND a good cut.
Regarding hair type & colour;
This can be solved by sending a hair sample.
Density can be gauged by supplying a good picture of the sides & back.
In reality a medium/light density suits most guys.
A graduated density at the front hair line helps with realism.
Correct ventilation is just making sure the hair lays in the same direction as the natural hair does, where the two meet.
This can be detailed when supplying a template, using arrows to show hair direction.
And then, a good cut.
A good cut helps in just the same way a good cut makes any hair look its best.
Sounds like there's a lot to think about but once you know your hair, you know your specifications and the job is done.

Its equally possible to buy a stock piece where all of the above ISN'T exactly the perfect specifications for you and with an ok cut and a bit of grooming, you can STILL make the piece work for you.
But if you want it to look as good as possible, go the custom made route and tailor it to exactly what you want.
The other tip I would ALWAYS recommend is to have AT LEAST 2 units (at any one time) to be on the safe side & to make your life easy.