My students loved these! I created them to be used with magnetic letters.
I found it best to print onto cardstock and laminate for durability. I purchased some cookie sheets from the dollar store and the paper fit right on top. This way, a student could place a card onto the cookie sheet and practice letter recognition and spelling of sight words with magnetic letters.
Personally, I picked out the letters they would need and kept them in the basket of materials for the center. This way there were not a bunch of uppercase letters mixed in that the students would not need.
The cards could also be used once laminated for students to practice writing the letters with a dry erase marker for older students!