I have followed the steps in the translation video in mirroring the foot. I had no problem with copying over the toe, roll or pivot, but, after you mirror the key poses, offset them in the action editor and then offset the feet by half of the body's Y movement to match back up with the cycle (As in, 1.3/2 in the necessary direction), I had multiple issues with this. On my first try, the mirrored foot kept contacting at different distances from the other foot (The Y graphs looked like they were uneven). The second try had the foot sliding on the ground when it should've been stationary and on this try, the mirrored foot slowly "moves" in front of the other and the graph's Y lines drift apart. Am I forgetting a step or have I done something wrong?
It can be a little bit tricky to get it the first time, but stick with it. Once you find the error that you are making, you will understand the whole process it a lot better. (Making mistakes is part of learning)
As to what the cause of the error is, I’m not sure exactly. Maybe you can post the issue in your submission or in the forum and @mention me so I can find it.
If you are translating the character forward (and not doing an "in-place" cycle) it is a lot harder because there is a lot more things that can go wrong.
I would suggest trying it as an "in-place" cycle first, before actually translating him forward.
That way you have one less thing to think about when animating it.