Here’s a post from SailfishOS.org on how to create Icons for your app and they compare it to baking cakes.
1. First, we need a background shape. Everyone screams for it, so let’s create one.
Base shape is problematic. One shape can be good for
some icons and bad for some others. Always it ends up with a compromise. Often it may also take more attention than icon metaphor visuals. Let’s build a system with multiple shapes. We are not only aiming for consistency but also for hierarchical cues
tailored for each icon metaphor visual.
Let’s pick the most popular shape: a rectangle.
3. up next: a circle. Let’s turn up the heat and merge them
And you can find all the directions on the pdf document. You can also download a template with different formats