If you are struggling, here the mapping for KING:
Plain a b c d e f g h i j k l m n o p q r s t u v w x y z
6 g h i j k l m n o p q r s t u v w x y z a b c d e f
8 i j k l m n o p q r s t u v w x y z a b c d e f g h
10 k l m n o p q r s t u v w x y z a b c d e f g h i j
13 n o p q r s t u v w x y z a b c d e f g h i j k l m
If you are struggling here is a Vigenère Calculator [Link].
The great advantage of this type of code is that the same plaintext character will be encrypted with different values, depending on the position of the keyword. For example, if the keyword is GREEN, ‘e’ can be encrypted as ‘K’ (for G), ‘V’ (for R), ‘I’ (for E) and ‘R’ (for N). To improve the security, the greater the size of the code word, the more the rows that can be included in the encryption process. Also, it is not possible to decipher the code by a frequency analysis, as letters will change their coding depending on the current position of the keyword. It is also safe from analysis of common two- and three-letter occurrences, if the keysize is relatively long. For example ‘ee’ could be encrypted with ‘KV’ (for GR), ‘VI’ (for RE), ‘II’ (for EE), ‘IR’ (for EN) and ‘RK’ (for NG) [Example].
Plain a b c d e f g h i j k l m n o p q r s t u v w x y z
1 b c d e f g h i j k l m n o p q r s t u v w x y z a
2 c d e f g h i j k l m n o p q r s t u v w x y z a b
3 d e f g h i j k l m n o p q r s t u v w x y z a b c
4 e f g h i j k l m n o p q r s t u v w x y z a b c d
5 f g h i j k l m n o p q r s t u v w x y z a b c d e
6 g h i j k l m n o p q r s t u v w x y z a b c d e f
7 h i j k l m n o p q r s t u v w x y z a b c d e f g
8 i j k l m n o p q r s t u v w x y z a b c d e f g h
9 j k l m n o p q r s t u v w x y z a b c d e f g h i
10 k l m n o p q r s t u v w x y z a b c d e f g h i j
11 l m n o p q r s t u v w x y z a b c d e f g h i j k
12 m n o p q r s t u v w x y z a b c d e f g h i j k l
13 n o p q r s t u v w x y z a b c d e f g h i j k l m
14 o p q r s t u v w x y z a b c d e f g h i j k l m n
15 p q r s t u v w x y z a b c d e f g h i j k l m n o
16 q r s t u v w x y z a b c d e f g h i j k l m n o p
17 r s t u v w x y z a b c d e f g h i j k l m n o p q
18 s t u v w x y z a b c d e f g h i j k l m n o p q r
19 t u v w x y z a b c d e f g h i j k l m n o p q r s
20 u v w x y z a b c d e f g h i j k l m n o p q r s t
21 v w x y z a b c d e f g h i j k l m n o p q r s t u
22 w x y z a b c d e f g h i j k l m n o p q r s t u v
23 x y z a b c d e f g h i j k l m n o p q r s t u v w
24 y z a b c d e f g h i j k l m n o p q r s t u v w x
25 z a b c d e f g h i j k l m n o p q r s t u v w x y