For the cardboard:
You will need a 5" by 41/4" piece.
Measure 1/4 inch sections and draw lines to cut the 1/2" slots for the string.
Place the string through the first slot and bring to the front and across the front of the cardboard to the opposite slot.
After the string is placed in the second slot, bring the string back through the adjacent slot giving it what I will call "bunny ears".
Repeat these steps until all of the slots are full.
Starting from the right side of your work, weave under-over-under-over-etc. to the end.
Leave the end long.
Weave back through from the left just opposite of the previous weave (Your new weave should never match the previous weave.)
(If you are making the striped coaster, this is the pattern you will want to follow)
**ALSO: Make sure that you are not pulling too hard on the strands, causing the outer strings to bow in. This will make your coaster funny shaped.
Flip your work over and do the exact opposite with the blue until it meets the other.