Multiply the first leg of a triangle by a given ratio. For example, consider a triangle that has legs of the following lengths: 3 inches, 4 inches and 5 inches. If you want to double the size of the triangle, you would multiply the first leg by 2. Therefore, the first leg of the similar triangle would be 6 inches long.
Multiply the second leg of the triangle by the same ratio factor. In this example, you would multiply the leg that is 4 inches long by a ratio factor of 2 to get a leg that's 8 inches long in the similar triangle.
Multiply the third leg of the triangle by the same ratio factor. In this example, you would multiply the 5-inch leg times 2 to get a leg of 10 inches for the similar triangle.