Split your string to words like described. Then create an empty array of lines and an empty temporary string. Loop over your words. In the loop, pop a word off the words array and append it to the temp. String. Check for the length of it. If it is more than line_length push the temp. String to the lines array and break the loop, else do nothing. Repeat this until your words array is empty (this means another loop around the described one).
While this is not as elegant as the solution posted before, it might be understandable by a novice programmer.
