Why did the founding fathers find it necessary for the constitution to be amended and what formal process did they choose?

When the Constitution was being drafted, the many of the delegates were in one of two factions: the Federalists and the Anti-Federalist. They had opposing views on many things, including how the Constitution should be interpreted, and what type of economy the country should have (industrial vs. agricultural). 

Anyways, the Constitution was basically a compromise between the two factions. The Feds (short for Federalists) feared that the Constitution wouldn't be ratified by enough states, so tried to persuade the Anti-Feds for a deal. They wanted to add some Amendments to the Constitution that gave more freedoms and rights to the people. Dozens of Amendments were proposed, but they were eventually narrowed down to 10.

These 10 Amendments became the Bill of Rights, because they gave many rights to the citizens of America.