〜ておく: preparing for what comes next
Finishing an action and preparing for a later need are not the same. With ておく, look for what is being prepared for or which state is to be maintained.
Keeping a later need in view
明日の授業の前に、この文章を読んでおきます。 means I will read this passage in preparation for tomorrow's class. Add おきます to 読んで, the て-form of 読む. The earlier reading serves the later class.
予約しておきました means a reservation has already been made in preparation. Since ておく changes to forms such as おきます and おきました, do not read every preparatory expression as future tense.
Leaving a state as it is
まだ使うので、その箱はここに置いておいてください。 asks someone to leave the box here because it is still needed. Distinguish the action of placing it, 置く, from ておく, which relates the placement to the later use.
窓を開けておきます。 may mean leaving the window open, for example for ventilation. If the reason is unstated, look at the surrounding explanation rather than deciding on a particular purpose yourself.
Comparing てある and conversational contractions
読んでおきます presents a preparatory action, while 印刷してあります can report the state of an already completed preparation. Similar translations do not make てある and ておく interchangeable in every sentence.
In familiar conversation, 読んでおく may contract to 読んどく. First connect the full form to its preparatory meaning, then practise recognizing the contraction. Choose the appropriate register separately for polite instructions.
Check: Does 予約しておきました。 mean the reservation has not been made yet?
Show the answer and why
No. It means the reservation has already been made for a later need. おきました marks the past; ておく does not automatically make the action something still to be done.