Anyone who managed to convert to mybb from other scripts?

[Serpius]

I understand you want to help but the description is not enough. I have read many who still have trouble converting. Then you say changes in .htaccess file- Do you know what file it really is? It's a file you do not touch any time.

That .htaccess file is not always required.

I know this because when you first install a new MyBB website, there is no active .htaccess file, but you can access the website and AdminCP without the .htaccess file.

Technically, you can run a MyBB website without .htaccess file, but it's wise to have a simple .htaccess file. 

In the instructions, it tells you to find the htaccess.txt file and rename this to .htaccess (dot must be at the beginning- remove .txt, then save the file) and upload this to your website files.

You are permitted to modify the .htaccess file as needed for your website.
It's wise not to make too many changes to the .htaccess because it might stop your website from showing up.