https://knsv.github.io/mermaid/
place the <$mermaid> widget and then use the mermaid syntax with in the text.
<$mermaid text="
graph LR
A-->B
B-->C
C-->A
D-->C
"><$mermaid>
An alternative method is to make the Tiddler type to be text/vnd.tiddlywiki.mermaid
.
You can also try putting the mermaid syntax within the <$mermaid> element, but this does not work as well due to TW5 parsing.
You can install by browsing to https://gt6796c.github.io/ and dragging the link to your TiddlyWiki.