Everything I know is from the site I mentioned at the beginning.
It is worth also to contact Polish embassy in Pakistan and ask which visa you should apply, give them all informations that you have about employer, place of work, what documents you posess etc. Ask about the insurance - how to get it from your place (you need it at the border)
Read here what you need from employer:
The employer prepares a statement containing: employer data, personal data of the foreigner the employer intends to hire, the start date and duration of the work and the type of work to be performed, the amount of gross pay (what is gross remuneration ->), profession and place of work ...
The employer delivers the original declaration to the foreigner; if the foreigner is living abroad, the document should be sent abroad
The registered declaration constitutes the basis for the foreigner to be granted