transit G type of china while traveling to Japan from Islamabad, Pakistan

Dear sir / Madam
My name is Mudassar Ahmad from Islamabad, Pakistan.holding Pakistani passport,I want to ask about transit G type of china.
I have three months valid visa of japan and i am planing to Japan in end of September this year,and want to stay in Beijing maximum two days, please advice how i can apply my transit G type of china, I will be very thankful to you

Regards
Mudassar Ahmad