〜たり〜たりする: examples, not a sequence
たり is useful for naming representative activities. Their order in the sentence does not necessarily tell you which happened first.
Build it from the た form
Form it as 読む → 読んだり, 書く → 書いたり, and する → したり. Do not simply attach たり to the dictionary form.
休みの日は、本を読んだり、散歩したりします。 gives examples of things done on days off. It does not mean these are the only activities.
The sentence ending carries the tense
Do not infer past tense from the た inside たり. 読んだりします can describe present or habitual activity; 読んだりしました describes past activity.
Learn the final する that gathers the examples into a sentence. Its polite form is します, and its past polite form is しました.
Distinguish sequence and choice
To emphasize an order, use a sentence such as 手を洗ってから食べます, “I eat after washing my hands.” たり alone does not establish a required first step.
AたりBたり is not an instruction to choose either A or B. A single たりする can also give one example, with other possibilities left to context.
Quick check: What does 昨日は買い物をしたり、料理をしたりしました。 tell you?
Show the answer and why
Shopping and cooking are examples of what the speaker did yesterday. There may have been other activities, and shopping did not necessarily come first.