If the grid is an 8x8:
To solve this we need to convert 3/4 into a fraction x/8 so that they have the same denominator.
To do this we can set a porportion and find a common denominator. 4 x 2 = 8 so the common denominator would be 8 and the rule would be x 2 so 4 x 2 = 8 and 3 x 2 = 6
So we get 7/8 and 6/8
From top to bottom shade 7 spaces in a row
From left to right shade 6 spaces in a row
Fill in the spaces in between to make a square
I hope this helps!