Well, percent ionization, simply allows you to find X which is the change in amount of substance into the final amount, it is dependent on the initial concentration of the substance present that is being dissociated.
For example, if the percent dissociation of CH3COOH is 1.5 % for an initial concentration of 0.15, then to solve for x, you would need to write percent as a decimal and solve.
% dissociation = X/Concentration of substance dissociated
0.015 = X/0.15
X = 0.0025.
Now since you know X, use the ice tables to solve for concentrations of species at equilibrium and plug in Kc expression.