Loops, arrays, and math. Remember that in many languages (C# included), strings are just arrays of characters.
For each letter in the input string, use the width of each character and the letter's position in the alphabet to find the indexes of the map elements you need to print. Example: if the letters are 3 wide, the letter A would be in positions 0-2, B is in position 3-5, etc. Repeat for each row.