Do these step by step walkthroughs actually help?
I don't mean to question the OP, it's great that he is trying to help, I'm merely curious.
If you play this game enough, you form strategies with your bike for every scenario. If there is a really long gap you bunny hop and try to land on your back tire and throw yourself up onto the edge, etc.
This game just seems way too fluid for any kind of walkthrough to truly help, you have to just keep practicing and learn how the bike reacts and how to manipulate that.
As for the achievement, probably the hardest (for me at least) was uphill struggle and magic marker or whatever it is called. But with enough tries you learn how to manipulate the bike, and so the next time around you manage the jumps no problem.