Exodus 26:10

And thou shalt make fifty loops on the edge of the one curtain that is outmost in the coupling, and fifty loops in the edge of the curtain which coupleth the second.

Cross-reference

Exodus 26:4-6 gives the same loop-and-clasp method for the linen curtains, which this verse applies to the goat hair curtains.

Exodus 36:12 records the actual construction of the loops, fulfilling the instructions given here.