Before You Start
Your rap is a lot more than just a bunch of words put to a beat. It is an extension and expression of who you are as a person.Your thoughts, beliefs, personality, and sense of humor, are all part of your rap and will influence how you go about writing. Before you start, take stock of these traits as they will have an impact on the choices you make when practicing the skills below. A simple brainstorming exercise will work, but the deeper you go in understanding yourself the easier it will be once it comes time to write.
Your Story - The Beginning
Content is what you talk about inside your rhymes. The topic matter. It may be anything from your personal experiences, to something somebody said, to something you dreamed up. Whatever you decide to rap about, ensure that it's something you know well. Attempting to write about something you are not familiar with or something you do not believe in will making writing difficult and could alienate your fans.
If content is whatever you talk about in your rhymes, flow is the words you select and the way you arrange those words to state it. Deciding which words to utilize or how to arrange those words is as easy as being attentive to the way you talk to others and in what way you communicate with yourself. What words do you use in a than others? What areas of a story would you emphasize that others don't? These differences are what make you unique and in rap, what enable you to get paid.
Time To Spit
You started using your subject matter, converted it to words and arranged them to a sheet of lyrics. Time for you to perform. Delivery will be the performance area of rap. It's everything from the tone of your voice, to the energy you bring, to the way you inflect certain words. You can be ironic, frustrated, laid back, paranoid, or straightforward. You could slur words together, or pronounce each syllable. Test with various styles and see which comes most normally to you personally.
Putting It All Together
As you probably already see, these skills are related. They depend on, reference, and expand upon one another. The way you deliver the words will depend on which words you select, which are influenced by what you may choose to talk about. You need to consider this relationship when writing. If you get stuck or aren't sure where we can get started remember, everything starts with you and what you are about so be yourself.