Aristotle has just finished explaining how similarities can help us construct definitions. He is now finished his discussion of the various tools of argument. He concludes his discussion of Book 1 of the Topics by explaining what he has done so far and what he is going to do next.
The instruments, then, whereby deductions are effected, are these- the commonplace rules, for the observance of which the aforesaid instruments are useful, are as follows.
The tools of argument have now been discussed. Next we have to discuss the common rules for how to use these tools to construct arguments.
Remember that Aristotle has discussed a number of topics in Book 1. First, he discussed the subject of dialectic in general. He explained what it was, why we learn about it and how we will know when we have learned it. Next, he divided the topics that could be discussed into four and explained what they were. He also explained how these categories related to identity and his categories. Finally, he started discussing how we would gain the right materials to argue with. He discussed dialectic principles and definitions in general and then discussed the four tools of dialectic.
He needs to discuss how to use these tools before going on to discuss how to argue particular kinds of subjects. Both of these things will come later.
Next, Aristotle discusses the difference between universal and particular problems in Book 2.