How did Saul summon Israel to rescue Jabesh-gilead?

Answer: He sent out pieces of oxen

Saul cut a pair of oxen into pieces and warned that the same would be done to the oxen of anyone who did not follow him.

“He took a pair of oxen, cut them into pieces, and sent them by messengers throughout the land of Israel, proclaiming, “This is what will be done to the oxen of anyone who does not march behind Saul and Samuel.” Then the terror of the LORD fell upon the people, and they came out together as one man.”— 1 Samuel 11:7 (BSB)