When you can summarize what you have written in your essay, you already know how to write conclusions with confidence. A conclusion is just an extension of your essay which concludes it by elaborating on the thesis statement and ends logically covering in short all that has been discussed through your essay.
The conclusion should be based on the established norms of writing good essays. If you know how to write essay introductions and how to elaborate the thesis statement in the body of the essay, you are already familiar with how to write good essays. There are some practical aspects you should take care of to really impart confidence to a reader about the quality of your essay.