Stuck on finding M2 knowing M1 in One Time Pad
If you know M1 XOR it to both sides of M1 XOR M2 = C1 XOR C2 which gives you M2


  M2=C1 XOR C2 XOR M1


Even if you didn't know M1, if it is long enough it (or M2) might have a common substring, i.e., a crib such as "the", "this". 

You might use the crib as an estimate of part of M1 to obtain a guess for a part of M2 by using the equation above focusing on 3 letter substrings. 

If you obtain as M2 a substring that looks like it could be English, you may have guessed part of the OTP key.