There are two writing tasks and you can use an hour to write both. The first is an analysis of a chart, a graph, a table or something similar. The second is a composition about an argument of general interest. It is suggested that you spend 20 minutes on the first task and 40 on the second. I always used more than 20 minutes for the first and about 30 for the second, but you have to decide on your own strategy on the basis of your abilities. Use your time in an intelligent way: remember you have to compose two different tasks and you can't devote too much time to the first (or the second). You can decide to start with the analysis or with the composition. It's up to you. However, remember to read both titles before you start writing. In this way your subconscious will start thinking about the second task while you are writing the first.
Now let me give you some suggestions about the tasks. First of all you have to write at least 150 words. This doesn't mean you have to count the words. The best thing to do is the following: when you write as practice before taking the real test, count the words. In other words, you have to know how long a text you write is. Don't write less than 150 words. However, a composition of 200 words will be perfect; a composition of 300 will be too long and boring for the examiner. In this task you have to present the data shown in a graph (or somewhere else) in a formal way. Try to understand the main points and write about them. In addition, try to explain the reasons of the trend shown: for example if the graph shows that in the last decades the use of cars has increased and the use of bycicles has decreased, you should say something about the possible reasons of this phenomenon. A possible organisation of the work could be the following: devote a paragraph to the introduction (say what you are going to write about); write a second paragraph describing the graph; use a third paragraph to explain the trends. Of course, this is just an example. The point is: you have to organize your work before writing. When you start writing you already have to know what you are going to write.
The suggestions about the number of the words and the organization are valid for the second task as well. The difference is that you have to write at least 250 words for this task. Use the first minutes to think about what you are going to write. Remember: ORGANISATION. You can decide your own way to write the composition; the following is an example: an introduction to write something general about the argument; a first paragraph to write the arguments of those who support a position; a second paragraph to write the opposite arguments. A conclusion with your own ideas about the problem. Try to avoid mistakes in grammar and make a composition in order: the paragraphs have to be clear. Leave a line between them.