You only need to know the rules you constantly forget and all you need is a summary. I take a piece of paper and block print tue name of each rule I really need to know.
When I first started to play White Scars I always forgot the following:
Scout
Hammer of Wrath
Hit and Run
That's all I wrote on my piece of paper printed in big font. After a few games I got it down pat.
You don't need a full description of each rule or you'll probably just get bogged down again.
Keep it simple... You'll be okay.