Measure a 6-foot length of log from the fallen tree, and cut it using a chainsaw. Try to choose a part of the tree with a uniform width. You will use this log as the base for the bench.
Insert two doorstops under the front and back of the log at both ends. The doorstops will hold the log steady and keep it from rolling. Glue each doorstop in place with wood glue, let dry and then secure it to the log using a hammer and three nails in each doorstop.
Choose four wooden chairs. They can be matched or mismatched, depending on your taste. Remove the chair backs. Try to unscrew them with a screwdriver. Or if they are not able to be unscrewed, cut them using the chain saw.
Line the chair backs up against the back of the log at equal distances.
Ask a friend to hold the chair back up to the log in the place you desire. Drill through the chair back and into the log. Use a screwdriver to screw a 3-inch screw into each hole. Repeat with each chair back.