Completing these strings is a matter of practice. If you practice, your muscle memory will store them in, and then it'll be pretty easy for you to use. The hardest are some of the strings from a selected few characters, like Raiden's LRC (DB1(Hold 1)xxRUN) and Jaqui's Stanky Leg (4U4).
If you check Tekken Master VS Sonic Fox Grand Finals in EVO, you'll understand why he's a guessing game. Tekken Master is a Kotal Khan mainer.
I happen to be a Grandmaster Sub Zero mainer here. If you need any help with him, hop on to Sub Zero's forums for any tips. If you like Sub Zero so much, check on Tom Brady and Madzin match ups.
Glad I helped a lot.