Step One: Aquire dog resadue (complete the paino puzzle, don't throw away dog resadue) (guides on how to do the puzzle are on the internet)

Step Two:
Go to Temmie Village (once agian, guide to find it are on the internet)

Step Three:
Use dog resedue to fill inventory with MORE dog resedue, sell all the items you got EXECPT 1 peice of dog resedue. Rince. Repeat. Profit.

Pictures are from google images and undertale.wikia.com