It's pretty easy! For example, for the word 食べる, you would write 食べました as the past tense of 'Eat'. Then make it continuous by changing the root word to the continuous word (食べて) and add いました。
＝＞ さっき、朝ごはん食べていました。(A little while ago, I was eating my breakfast.)
＝＞昨日、すごい映画を見ていました。(Yesterday I was watching a great movie.)
I hope my answer was helpful to you!