One way, without starting with a blank slate,would be to read a lot of spells, choose as many as you like, from which you borrow elements to write your own. You simply include your intended as the target.
If you are new to magic, it might be easier to find a working which best seems to fit your means and desires, and modify it according to your specific wishes.
Of course, you could always start from absolute scratch on the project, with listing aspects of the working you may eventually do. Consider how you prefer to work, materials you have at hand or can easily acquire, and other correspondences you may wish to include.
As Alan Chapman writes:
1: Decide what you want to occur.
2: Ensure that what you want to occur has a means of manifestation.
3: Choose an experience.
4: Decide that the experience means the same thing as what you want to occur.