Go slow and steady STOP when it hurts.Like blueboy and Beargizmo said :use plenty of lube ,dont force and keep it clean
use plenty of lube like jim said but don't force anything in. if the pain is unbearable don't force because it can cause serious damage
I personally believe the anus was not meant for sexual activity...it does not expand to accomodate a penis or a toy as the vagina does, and you can cause serious injuries to that part of youf body.
If you insist on such activity, plenty of lube and foreplay is advised. And I'm not sure the anus is capable of accomodating 8" or more. And make sure the "toy" is cleaned very well before inserting it into the vagina.
Jim