What sort of problems are you having?
What tool are you using to collimate?
What makes you think the collimation is bad?
From what I understand most of the dobs are done the same way so this may be helpful.
How to Collimate an Orion Dob using a Collimation cap or laser
https://www.youtube.com/watch?v=YAVGcGEBmCE
I use a collimation cap. I had purchased a laser but did not liked how it worked so I sent it back.