In a cover letter you have to include a few things
01. Hi/Hello (optional)
02. Who you are in short
03. Summary of the project / offer so that buyer understand you read that.
04. Your skill related to the work
05. Your experience related to the work
06. Your commitment to make buyer confident
07. Ask one or more questions to make buyer engaged about you.
Try to make your cover letter brief and no spelling mistake.
Hope you will get good result.