Here's a very comprehensive list of adverbs of frequency for your ELT classes.
In the activity, you can instruct students to work in pairs and try to complete the list according to their frequency. There are many grammar references that indicate all the adverbs with a specific per cent. I don't think it's a proper way to classify the adverbs according to a particular per cent, so I've just pointed always and never as %100 and %0 and left the rest empty. All the adverbs are listed on a scale from %100 to %0 relative to their frequencies, which I believe is a better way of classifying them. However, I also have left the per cent boxes empty for you to clarify the frequencies of the adverbs further if you want.
Below, you'll find the worksheet version, the whole set in a certain order that will help you go through all the adverbs of frequency and also a suggested answer key for your convenience.