This is how:
From Article I, Section 2:
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.
Translation: Slaves will be counted as three-fifths of a human being.
From Article I, Section 9:
The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, but a Tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person.
Translation: the slave trade will be permitted until 1808. That, by the way, was one of the terms that the patriotic, God-fearing representatives from South Carolina insisted on.
From Article IV, Section 2:
No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, but shall be delivered up on Claim of the Party to whom such Service or Labour may be due.
Translation: the Free States are obligated to return any fugitive slaves that run away to escape the brutality and barbarism of slave owners in the patriotic, God-fearing southern slave states.
Yes, the original Constitution was deeply and profoundly flawed. It sanctioned the hideous institution of human slavery, at the insistence of Southern conservatives. Slavery, which should have been abolished outright by the Constitution, was not only preserved, but the other states were compelled to support it by being forced to return "property" that had fled to them and recognize slaves as partial humans. An appalling, disgraceful phenomenon.
By the way, you'll notice that the words "slave" and "slavery" were not used in the original Constitution. The writers didn't have the courage to actually say what they meant outright, preferring to hide behind euphemisms. The word slavery wasn't used in the Constitution until the 13th Amendment, which outlawed it in 1865.
Yes, slavery was outlawed after a terrible war that had devoured 620,000 lives, a war that was started by traitorous, violently anti-American slave owners looking to preserve their barbarous system--a system that shouldn't have even existed in a country founded on the stated ideals of the United States, a system that was codified into law by the original Constitution. I call that flawed.
Who would dare to disagree?