how to going to terminal 1 from terminal 3, PEK

I will arrive at terminal 3 & have to connect flight to Weifang via Hainan airline at terminal 1, but i this is 1st time for me to BJ & connect flight, so i quite worry about the way that, how to going from terminal 3 to terminal1? would you kindly let me know? how to be there? because i have time limit to connect flight only 2.5 hrs.