I have gone through this before. I'll share with you what I did to tame the nazal sounds.
Make sure the mic is away from the sinus area. Have it a bit lower, maybe aimed at the person's chin area.
Then ask them to hold their head up a bit and instruct them atht they are singing just above the capsule. Maybe give the person something to look at and keep[ the pop filter up as it should be.
Also, make sure the mic is back a bit, otherwise if they start to sing down into the mic you'll really hear the nasal sound. With the mic further back it equalizes the sound source a bit. By further back I mean about 1.5-2 foot or so. Not real close about 1-6 inch like some people do it.
Also take the time to listen to a a few of your mics to choose what is best suited.