Sorry if this is too detailed, but here is how I do it. Put a 12" piece of 2x8 between your jack and your oil pan. Jack up your engine slowly until you can remove the blot that connects your engine frame bracket to your motor mount on one side. You need to jack it up just enough to take pressure off the bolt. Once you have removed the bolt, remove the two bolts that connect your motor mount to your block. Lift your engine about one inch more and remove the mount. Install the new motor mount and the two bolts that hold it to the block. Lower the engine down slowly until you can replace bolt that connects the motor mount to the frame bracket. Lower it down all the way and torque everything down. Once you have everything back together, do it on the other side. It is easier if you don't remove both mounts at the same time.
I know you have a 68, but for those '67 289 owners reading this, you will need to get '68 style frame brackets to use after market motor mounts. If you don't, your driver side motor mount will break in half.