In German we would often use the phrase "In einem ersten Schritt" which literally translates to "In a first step[, we did this or that..]". It is often used when the order of steps is a bit arbitrary.
This is in contrast to "Im ersten Schritt" ("In the first step") which is often used when the order of steps is more obvious or natural.
Now Google Doc always suggests that I should change "In a first step" to "In the first step" and I wonder how native English speakers fell about this. Does "In a first step" feel right to you under the conditions mentioned above?
Example: In a first step, the police officers examined the alibis of possible suspects. In a second step they searched for witnesses in the neighborhood. (here, the second step does not necessarily depend on the first step and there would be multiple possible first steps)
Here's a similar, but not identical question: