How do you use a few in a sentence?

You mean, right next to each other, like in this sentence: "They had to remind her that that is not allowed."

Yes, you can. It definitely sounds awkward, and most professional writers will try to avoid it whenever possible. But I don't think it is considered to be grammatically or stylistically incorrect.

Same goes for other Words, like "is": "It's not dangerous, and anyone who says it is is simply misinformed" or "They had had no food or water since the day before yesterday."