Hi, Dave. I'm sure that there are some posters on this board that are better at it than I am but I will give it a go. Those that are, feel free to correct, modify, amend to tell me I am totally wrong.
Start by wet sanding with 400 wet/dry paper and a sanding block. You want to level the surface and feather out into the surrounding area. You do not want to go all the way thru the poly that is already down unless you can't help it. Wet sanding will remove the old poly about twice as fast as dry so be careful. You do want to leave some very light scratches for the new poly to bond but this will look more like a milkyness/flatness than actual scratches. You will have to wipe away the sanding gook every so often so you can tell what kind of surface you have. When it looks right, rinse well to get off all the gook. Again, don't oversand, just get it smooth.
Try to use the same poly that your contractor did. And use a good brush--the poly can should tell you what kind, but probably natural bristle and no more than a 1" brush unless you scuffed up a really big area. Pour some of the poly into a separate can and add mineral spirits (I assume that it is oil based poly) to make a 50/50 mixture. Since it will be real light, kind of feather/float it into the sanded area. Wait 2 days and repeat the whole process, sanding very lightly and mixing the poly 75/25. Wait 2 days and repeat the whole process mixing the poly 90/10. Try not to get the new poly into any of the old poly/unsanded area.
That should pretty well get it. Be sure to stay away from the new poly and don't touch it while it is drying. Have a cup of coffee when you are done and congratulate yourself on the great job.