Mentally (or on paper) divide the acid into H⁺ ions and negative ions. Similarly, divide the base into positive ions and OH^- ions. For products, the H⁺ and OH^- ions make H₂O.  (At this point it may look like you have an excess of H⁺ or OH^- ions - don't worry that you don't have enough of both to make a whole number of H₂O molecules!  When you balance the equation you will find that there will be plenty of each ion in the end.) So the next step is to combine the positive and negative ions to make an ionic compound (a salt) taking into account the charges on each ion. In this case, Mg²⁺ and Cl^- make MgCl₂. If the formula of the salt is correct, you should have no trouble balancing the equation and you'll see that you do have whole numbers of water molecules.