There a huge number of websites out there sharing all types of information about sharpening forstner bits, I have tried a few and this is what I find works best.
I bought cheap forstner bits that were titanium coated for extra edge durability. As the bits didn't arrive very sharp there is no way of confirming if the titanium coating helps with edge durability.
After the first use the bits were dull, burnt and chipped. I was drilling in to Iroko at a sensible rpm but for a new bit that shouldn't have be a problem.
To sharpen I use a rotary tool such as a Dremel or pictured is a black and Decker one. A cut off disc used for the flat area's and a cone shaped stone for the rounded area (see picture). The important thing is remove the same amount of material from each surface so the bit still works as intended.
This only takes a few moments and removes the titanium coating, but the bits cut better than new.
There are a few variations in forstner bit design but it you use the disc of flats and the cone for curves you should be able to do them all.
Once grinding is done a quick de-burr with a honing stone or mop maybe needed to take off any wire edges.