There are some videos for fixing mistakes on the ...And More tab on basic techinques (here
As for a life line. When you finish a row & you know everything is correct you can thread a piece of scrap yarn, dental floss or fishing line, into a tapestry needle and then thread it through the stitches on the needle. Be sure to only to through the open spaces of the stiches (don't go through a piece of yarn. You can leave the life line in for as long as you want to and then just pull out gently when ready to insert the next life line.
You can also use extra cables from an interchangeable set, if you have them, as a life line.
I hope this helps - let us know if you have any other questions.