I'm a Newbie

First, sorry for my English, because I'm a pupil, 16 years old, from Germany. There are no good sites in German, but this site in English is very good. That's why I'm scribing here.
My problem is how to fold that "Medusa aurelia"
Look here: http://design.origami.free.fr/Sirgo/diagrams.htm
I have problems at step eight, I mean from step eight to nine.
I don't know how to use the thick arrow and the black thin arrow.
I don't accomplish how it looks in step nine.
Can anybody help?
I'm despairing.
I hope you will write soon
Greetings
Sandy