In order to get the top marks (7-8 marks) for the essay, you need to have a balance between outside knowledge and the sources. Using ONLY sources or ONLY outside knowledge will only get you about 4 marks if you do everything else perfectly. This means that you should have quite a few points where you bring in your outside knowledge on the topic, but you should have roughly the same number of points from the sources in your essay.
I structure my Paper 1 essays like this:
1. Thesis statement. Here I state my answer to the question. To earn the most marks, you need a multi-causal answer (meaning just stating one little factor won't get you very many marks, you need to list a few things, usually two or three). For example, if your question was "Explain the factors that led to the outbreak of World War I," you wouldn't get many marks by saying "World War I was caused by nationalism." You'd get much more by saying something like "World War I was caused by a combination of intense nationalism, increased militarism, European imperialism, and the alliance system."
2. I write a good, solid paragraph proving my first point. Sometimes my point comes only from the sources, sometimes it's only from my outside knowledge, and sometimes it is a mixture of the two. If I use a source, I will quote a line from the source.
3. Then I write my other paragraphs.
4. Then I write a short conclusion. It doesn't have to be huge, usually just a few sentences. I sum up what I have discussed so far in two-three sentences, and then I show its importance, whether it led to something else, what impact it had on international relations, etc.
Obviously you should structure your essay based on how you write and how you organize things, but this is how I was taught to approach this essay. I do this nearly every time and I have not received lower than 7 marks on the essay.
I attached a mini-response question I did a few months ago in class if you want to see what I'm talking about. It earned 7 marks, with the only suggestion just a few more details.
Good luck on your essays