Calculate the mass of the solute in the solution :
Molar mass KCl = 74.55 g/mol
m = Molarity * molar mass * volume
m = 0.9 * 74.55 * 3.5
m = 234.8325 g
To prepare 0.9 M KCl solution, weigh 234.8325 g of salt in an analytical balance, dissolve in a beaker, shortly after transfer with the help of a funnel of transfer to a volumetric flask of 100 cm³ and complete with water up to the mark, then cover the balloon and finally shake the solution to mix
hope this helps!