Generally and simply, add 了 behind the verbs or the sentences. e.g. 我吃饭了，我吃了饭。 or add 过 behind the verbs or with a 了 at the end of sentences. e.g.我吃过饭了，我吃过饭。

1 use "time word" in the sentence, like yesterday, last year, at that time etc 2 use "result word" after the verb -- different for different verbs 3 use le 了

Past in Chinese is a bit more than just adding "time related words" . The completion of an act: Verb+了. When a verb is followed by 了 it indicates the act is completed ie: Q:你喝了吗？ A：喝了 If the "verb+了” takes an object a "number quantifer compound" or other word is required before the object: ie: 我买了三本书。 If the object does not have a number quantifer compound or other attribute before it, the particle 了 must be added at the end of the sentence to make it complete and this sentence is used to relay certain information to the listener: ie: 我买了书了 (我不买了。/你不要给我买了） If the object doesn't have a quantifier or attribute or no 了 at the end then another verb or clause must be added to show a second act followed the first one. 昨天，我买了书就回家了. ( a little more involved than just adding yesterday to the sentence) *NOTE： in Feng's example 我作业做完了。 This 了 is used with "the grammar point " complement of result" 结果补语 where verbs such as 完，懂，见，开，上 and ADJs 好，对，错 can be placed after the verb to show the result of the act. for example: 我听懂了老师的话。I understood what the teacher said. 我吃好了。 I ate well.

there's no tense in Chinese at all. Just to use the time-related word, such as yesterday, to express that point.

there is no past tense nor past participle in Chinese, to express something already happened, you simply put yesterday or similar nouns in your sentences.