Addition modulo 26 in Digital Logic
You should be able to do it with three 4-bit adders, a 4-bit, 2-input mux and an OR gate:

Use two of the adders to form the 6-bit sum of the original two numbers. If the MSB (bit 5) of this sum is set, you know you need to subtract 26 from the result.

Here's the trick: Doing a %26 operation on a number is not going to affect the LSB (bit 0), so the LSB of the sum formed above is already the LSB of the final result.

Use the third adder to add the constant 0011 (the decimal value 6, with the LSB dropped, too) to bits 4:1 of the first sum. If the carry out from this adder is set, then the original sum was 26 or greater, and you need to subtract 26 from it &mdash; which is what the output of this adder is!

OR together the carry bits from the two adders and use its output to control the mux. The mux is going to select bits 4:1 of the first sum if the OR gate is low, or bits 3:0 of the second sum if the OR gate is high, to become bits 4:1 of the final result.

I hope that is clear; if you need a diagram, let me know.