I would like to start by saying that 45 minutes is not a lot of time to write up a film review for a student at pre-intermediate level. Consider how hong it would take you to do it! So be realistic when setting your aims and don't expect too much from them.
Guided writing consists of taking students through the stages of writing. You would usually start by focusing on the structure of a piece of writing, so your use of an existing film review is a very good idea.
Here are some activities you could use. You can pick and choose between them to tailor the lesson to your students. You'll need to consider their level, how much challenge they can handle as well as their personal interests. You can also spread the work out over two or three lessons and / or set some of the activities for homework. You might also want to spend some time in a previous lesson on useful vocabulary.
A general outline would be:
1) focus on the structure of a film review
2) plan the film review
3) write it up
1) focus on the structure
Choose a film review that is at a slightly higher level than the students could produce themselves. Remember they can understand more than they can produce. To draw their attention to the organisation, you can do one of the following:
Cut the review up into strips that the students re-organise. The strips can be single sentences, a cluster of two or three short sentences, cut in the middle of a sentence where the link is obvious or cut up randomly.
When planning the lesson, look at how the review is organised. For example: background, details, personal reaction, a recommendation to go and see it (or not). Cut the review up into the sections you have identified (ie all sentences describing the personal reaction are together). Hand these sections out in class and write the titles of the sections up on the board. Students match the title to what they have in front of them and then decide in what order the sections should go.
2) Plan the film review
Allow students to decide which film they want to write about and then group them according to their choices. This will be quicker than putting them into groups and then allowing them to choose the film. They brainstorm elements to include. They then choose the ideas which seem most relevant and decide how to organise them, considering the categories that have been identified previously.
3)Write up the film review
Students can either write in groups or take one section each and write it up individually. If they write it individually, it will be less time-consuming but they'll need to bring it all together into a coherent whole.
To give students more guidance, you could use pre-printed sheets showing the beginnings of paragraphs. Students complete the sentence in a way they see fit and then continue the paragraph. This can be less daunting for students who are "allergic" to writing.
You can look at my answer about process writing for some ideas on how to go about phases 2 and 3.