I20 需要second year support

发现自己在提交 I20 申请时忘记交一个表：
"If your academic program is longer than 12 months, you must provide a plan for covering future estimated expenses. An acceptable plan for covering future expenses includes 1) A signed letter explaining specific personal funds to be used, or 2) A financial sponsor indicating future support on the affidavit of support."

这个signed letter，怎么写？因为自己的项目是 16个月，自己就简单提交了12个月的财产证明。。。