This is a super cheap, super easy project that results in a super cool product :)
Slate £ free (offcuts, speak to your local roofer)
Foam 75p per A4 sheet (most art / craft shops)
Total = 50p per pack of 4 tiles....... that's 12.5p per tile!!!!!
Step 1: Mark out slate
Use a ruler and a stick of chalk to mark your coaster