This is the 8553rd most frequent Japanese word.
挟み込む
To insert, squeeze, or wedge something in between.
Here, 挟み込む is used to describe the action of inserting an object (bookmark) into a defined space (book).
彼は本に付箋を挟み込んだ。
He inserted a bookmark into the book.
In this sentence, 挟み込む metaphorically describes incorporating a concept or step into an existing framework (plan).
私たちは計画の中に新たなステップを挟み込むことを決めた。
We decided to insert a new step into the plan.
Here, 挟み込む is applied to the physical placement of an item (document) between other items (files).