I would consider the issue of overall conciseness. The active voice is fine; however, you could consider using the passive voice, which is very common in scientific writing, especially in the Materials and Methods section. It avoids many repetitions of the pronoun indicating who is doing the steps and puts the focus on the steps themselves.
For example
The solution was poured into a special container, allowed to reach room temperature, and then frozen at -7°C.
unless, of course, there is something particular or unusual about any of these step. (If others may adopt or repeat your process, you may want to describe or explain in what way the container was "special.")
If you are explaining in more detail a previous step, you could make the connection with Specifically, the solution was..., as suggested in another answer.
There is nothing wrong with the active-voice version: We poured the solution into a special container, allowed it to reach room temperature, and then froze it at -7°C, but I think being concise actually helps comprehension in cases like this. An advantage of the passive voice is that, in the long run, it can save space, which is a major consideration in journal publication. In a report, this could free up room for text or a figure you are leaving out for space considerations.