How do you use 的 at the end of a sentence
看足球赛 is a complete sentence

Adding adjective marker "的" after a sentence or phrase turn it into an adjectival phrase. 

Adjectival phrase describes the noun that follows.   

[Simple phrase]: 看足球赛 = watch football match

[Adjectival phrase]: 看足球赛(的) = (those who) watch football matches

[Adjectival phrase with subject]: 看足球赛(的)学生 = (those/ the) students (who) watch football matches

Since 看足球赛的 is an adjective, you can follow the grammar structure of [subject + is + adjective] and write 
"那些学生 是 看足球赛的" (those students are the ones who watch football matches)