Well I want to count the number of letterers in the sentence and then compare each against the input, and eventually output the result.

-define the location of the start of the sentence
-define the location for the count of characters in the sentence.
-define the location to put the letter to be searched for.
-define the location of the result

So I need to input the letter to be searched for, or load the memory location of the first letter of the sentence in a register

Then I guess I should create a loop to count the number of letters, stop the count at the period or carriage return then save the result.

Afterward I need a loop to compare the letter i'm looking for (upper and lowercase) against a letter in the sentence, while simultaneously incrementing two counters.

And i'll also have to take into account what to do if a condition is not met.

I'm just fuzzy on writing the actual code to accomplish these, especially the loops.