Starry, here's my suggestion:
Place boards, sticks, or whatever on the ground and place the log on top of these. The sticks will help to keep your chain out of the dirt. The pith should be horizontal.
During the cut, the log will be pulled toward you. You must prevent this. One way is to drive two short stakes in the ground next to the end of the log closest to you. Also, chock the log so that it cannot roll side to side.
Start the cut on the far end of the log. To prevent the chain from pulling the log up and toward you, have the bar/chain very nearly parallel to the surface as you begin the cut. Then, once you've entered the cut an inch or so, gradually raise the body of the saw so the bar will angle downward, eventually ending up at about a 45 degree angle to the log. The required angle will depend on the diameter of the log.
Once you have the cut established, simply continue it toward you until the cut is complete. Watch for any tendency for the log to lift. Once you get part way down the log, you can lower the body of the saw so that the bar/chain is more nearly parallel to the pith.
If the log is more than about 12" in diameter, you may have to make a cut on one side, then roll it over and make the final cut from the other side.
Email me if you are not clear on this. Be careful. ~Doc