can someone please show me and explain this to me, I am very confused. I will be using cryptool to do it..

PART-02: Encryption using binary/byte addition by using CrypTool-1 Under this encryption algorithm, the key entered is added character by character (byte by byte) to the data to be encrypted. Here addition modulo 256 is used, i.e. so that any carry-overs are ignored. The key is applied cyclically (as under the Vigenre encryption algorithm and also with the Exclusive-OR), i.e. once all the characters (bytes) of the key have been used, the algorithm reverts to the first character until the text has been completely encrypted. To decrypt the text, the characters of the key have to be subtracted from the encrypted text modulo 256. If one knows the characters which occur most frequently in the plaintext, it is then possible to work out the key with the aid of a computer (and hence also the plaintext) (see Automatic analysis, Byte Addition). The key used for Binary Addition is entered in the Key entry dialog. This encryption algorithm can be easily broken with a Ciphertext-Only attack (see Automatic analysis, Byte Addition). An example of this will be found in the Examples chapter