Outline
In order to create unique cipher characters, we must use a multiplier which is co-prime (the values do not share any factors when dividing) in relation to the size of the alphabet (26), so you should use either 1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23 or 25.
For example if we use "abcdefghijklmnopqrstuvwxyz" and a multiplier of 3, gives "adgjmpsvybehknqtwzcfilorux"
To help you, here are the letters with numeric values:
a - 0 b - 1 c - 2 d - 3 e - 4 f - 5 g - 6 h - 7 i - 8 j - 9 k - 10 l - 11 m - 12 n - 13 o - 14 p - 15 q - 16 r - 17 s - 18 t - 19 u - 20 v - 21 w - 22 x - 23 y - 24 z - 25