HOW TO WRITE
Don't make a fancy design for your newsletter, it doesn't convert. Keep it simple, personal and to the point.
THE RIGHT PLATFORM
You might choose a platform in the beginning because it's free, but be aware it can become expensive in the long run.
RE-USE YOUR SIGN UP FORM
Use multiple newsletter forms on your website. Place them multiple places, but make it feel natural. Analyse which forms works best on your target group.