Try writing a trial message of a few lines. make a deliberate error if necessary. To correct the mistake tap the screen just between the letter to be corrected and the next one. A curser should appear between the letters.
On or near the keypad there is a reverse arrow type symbol. Tap that and it will delete the offending letter then you can correct it.
I had the same problem with the manual......it assumes too much. Patience and trial and error is a must. Bottom of keyboard (on my Samsung)is the equivalent of the return key to go down a line.
Just play and practise, you won´t break anything.