For assumption,you’ve installed virtualbox and have virtual drive on your pc.
Ok.lets start :
1.open your virtualbox
2.create new virtual machine
3.give a name for your virtual machine
4.set base memory size
5.create new boot hardisk
6.click next to virtual disk wizard
7.select hardisk storage type
8.select virtual disk location and size
9.finally,click finish to start backtrack on virtualbox
10.now,go to setting your backtrack iso format,click setting
11.select cd/dvc room to mount backtrack image
12.add your backtrack image file
13.then,click start to begin backtrack
14.boot is running
15.backtrack is running,have fun