Using 〜ておく, in preparation of doing something.

おく is the verb you use when you want to express that what you are doing is in preparation for something else, or doing something in advance. It is always used with a verb in て form before it, and is often paired with a 前に clause.

Examples:
パーティーの前に、食べ物を買っておきます。
I bought food before the party (in order to prepare for it).
卒業式の前に、しょうたいじょうをおくっておきます。
Before (my) Graduation Ceremony, I send out invitations (in advance).
電車に乗る前に、きっぷを買っておきます。
Before riding the train, (I) buy a ticket (in advance).
クラスに行く前に、宿題をしておきます。
I do my homework before going to class (to prepare for it).

There are also casual ways to say "ておく", these are とく and どく.
とく is used when the て-form of a verb ends with て.
Examples: たべておく→たべとく、買っておく→買っとく
どく is used when the て-form of a verb ends with で.
Examples: 飲んでおく→飲んどく、読んでおく→読んどく
Since this is a CASUAL way of saying ておきます, you will never use とく or どく in ます form, it just sounds weird.
Here are some examples:
友達が来る前に、へやをそうじしとく。
Before my friend comes, I clean my room (in preparation for their arrival).
しけんの前に、きょうかしょを読んどく。
Before the exam, I read the textbook (in preparation for it).