Don't despair. People on this site have taken scores from the 300's and turned them into 600's. I think there was one guy who went from 500's to 780, though I cant find his post now. Another went from 540 to 680. There's a few that went from low 400's to mid six's too.
An
error log is simple really -
You simply track which questions you get right and which you get wrong. However you want to do this is fine - excel is the easiest in my view, but you can do this in word or on a piece of paper. The idea is that you force yourself to come back to the questions you got wrong and
understand why. This is especially true if you keep getting the same kinds of questions wrong.
The most common "mistake" I think people make - I was guilty of it too - is that you see a question, struggle with it, end up with an answer and it happens to be right, so you patt yourself on the back and move on. Problem is, you may have stumbled on teh right answer the wrong way. Similarly, a lot of times people will look at question, struggle, give up, and go look at how they solved it. They'll look an say "ah ok, yea ok, that makes sense." and move on. You really have to force yourself to stop and look at the answer and figure out why they did each step. Don't forget to track questions you get right but feel were impossible. If you spent ten minutes on a question and got it right - good - but not good enough.
Before you can realistically tackle timing, you need to start getting more right - once you are getting more right, start worrying about timing. If you try and do it together you will (a) get the problem wrong and also (b) not absorb anything from the problem because you did it so fast.
If you are struggling with the concepts on the GMAT - I.e. rate problems, geometry, sentence correction, I can't recommend these guys enough - buy the
Manhattan books. If you don't buy any other book, buy their SC Book.