Write the symbols of the elements involved. For this example, the initial element is uranium, whose symbol is U. The atomic number is 92, and the mass number of this isotope is 238. So, write a U with a 92 to the left and slightly subscripted, and a 238, superscripted to the right. Then, write a Thorium symbol ("Th"), with a 90 subscripted to the left. To the right of the thorium symbol, write the symbol for alpha decay (the symbol for Helium, with a 2 subscripted to the left and a 4 superscripted to the right). Then, write the words "gamma rays" to the right of the alpha decay symbol.
Draw an arrow between the uranium symbol and the thorium symbol, pointing toward the thorium symbol. This shows that the uranium is transforming into thorium and the other byproducts.
Write plus signs between the thorium and helium symbols and between the helium symbol and the "gamma rays." This shows that all three items are by-products of the reaction. Subtract 4 from 238 to get 234, the mass number of the by-product isotope of thorium. Write "234" superscripted to the right of the thorium symbol.
Check your work. The subscripted atomic numbers should equal on both sides of the arrow (92 = 90 + 2), and so should the superscripted mass numbers (238 = 234 + 4).