You can buy pre cut letters but it is cheaper to make your own.
On a computer type the letters that you want and select the font and size. As these will be used as a template you only need one of each letter or number.
Then print out.
Cut out each letter and number to use as a template. On your cardboard draw around each letter or number with a pencil and cut out from the cardboard.
Once you have all your letters and numbers cover them in your chosen patterned tissue paper or paper napkin. Using mod podge to glue the paper on. This can get fiddly depending on the size of your letters. If you're using paper napkins remember to just use the one layer that has the pattern on, I found tissue paper easier as I didn't have to separate layers.
Once the letters are dry lay them out on the backing card and work out where you want them positioned. Then glue them into place.
Leave to dry and then put it in the box frame.