Even if there are basically no limits to your imagination, there are still a few things that you should be aware of. The visible streaming area is 1920 pixels wide and 1080 pixels high, which is why you should also create your layout in an image editing program with these dimensions.
Set the background to transparent so that the actual content can be seen later in addition to the Twitch overlay. Using the screenshot of your game or video, you can control how much space you have to plan for around the edges and then gradually insert the elements that your followers will see during the stream.
Remember that fonts and images should quickly attract attention and should therefore not be too small. Now you can start creating your very own Twitch overlay. Take your time and try out which design you like before introducing it to your fans and new viewers. When you have found your perfect overlay, save it in PSD and PNG formats. With streaming software such as OBS or XSplit Broadcaster, you can then easily integrate it into your Twitch streams.