I saw a similar idea and immediately had to make it.
Bought the white terracotta pot (I wanted natural but this ended up working out great) at Dollar Tree
I painted it a based yellow color so when I glued on the sand the white would not show through
Use matte Mod Podge and adhered the sand to the pot. I picked up a bag of sand at Michael's for 2.99 - 40% off coupon of course!
Added the twine also from Dollar Tree and hot glued the shells from an old Florida trip.