By watching this YouTube Memories video, you will be able to learn how easy it is to add a memory in any given Timeline.
To start adding your contributions to a Memories Timeline (already a Contributor), you can simply click / select the "Memory" button on the bottom left section of the page.
The 'Add a Memory' window will pop-up. Enter the Memory details, try to be as specific as possible.
Then finally click, '+ Add a Memory' to complete this process.
However, if you are not yet listed as a Contributor, please refer to this article: