Can't get donut gasket to seal on 88 ford f250?

Just replaced the exhaust manifold and now it's leaking exhaust through the donut, I can't seam to get it to seal properly, am I supposed to hammer it into the pipe? Anybody done this before have any tips?

No hammering. Clamp on by the bolts. Compare the old to the new and see if the new is the correct one.

As Kenny said be sure you've got the right gasket, then tighten the bolts alternately a little at a time until you get to the proper torque. The gasket may not be seated properly due one bolt being tightened more than others.