What not to do ?
Well thats an odd question that can be answered filosofically, and I dont see how can that benefit You.
Dont slack, dont procrastinate,, dont go thru HA300 examples just like going thru motions without deeply understanding how it works.
Also, I dont know your baseline knowledge, maybe you know halph of it allready.
For instance 3/4 of HA300 was old news to me, so I just browsed thru it. I took HanaSup certification before HanaImp so HA100 was old news to me and all Admin stuff.
I played with Hadoop allready before so that too. 20 years of programming experience and 15 of writting SQL procedures and SQL optimizing helped.
BI on the other hand is not what I worked with in Sap (and there's a lot of it, terminology and stuff) so I struggled at first a little bit.
Do Not not take Learning Hubs HA300E and HA350E, cause it will make your preparation easier (if youre not already heavilly working with Hana every day).
What I see lately around is that Abap people underestimate SQL. Hana SQL is not Abap SQL level knowledge.
So do not not learn SQL deeply, working with sets (thats not as much for certification but for good coding)
Also, I posted a story about my HanaSup certification a while ago, so maybe that can help you.
So do not not read that too 
HANASUP certification experience
Cheers and good luck !