Short version
Replace the pound signs in the following URL with the cluster number of the article of interest https://scholar.google.com/scholar_alerts?view_op=create_alert_options&hl=en&alert_params=hl%3Den%26as_sdt%3D2005%26cites%3D####################%26scipsc%3D
.
Long version
None of the other couple of answers worked for me, so I found a new way of doing this. It involves finding the cluster number of your article, as described by @Calimo in the other answer and then modifying the alert URL of an already created alert.
Click the Save button under a search result that is pointing to the paper of interest (this adds that paper to your library)
Open My Library, click on the article, and then scroll down to find "scholar articles" - that link (but not the one at the top) contains the 20 digit identifier you need. Copy this link and paste in into a text editor. The number after cluster=
is the one you want.
Search for an article that you know has citations and click "Cited by".
Scroll down to the bottom of the first page where there is a little mail icon that says "Create alert". Right click and copy this URL and paste it into your text editor.
In the alert URL, replace the number that comes after cites%3D
, with the cluster number from your article of interest.
Copy and paste this new URL into your browser and a page will appear asking if you want to create an alert for this article.
While it seems like the final URL always follows the same format, I outlined the entire process in case that URL would change in the future.
Example
Step 2 URL
https://scholar.google.com/scholar?oi=bibs&cluster=3568708134260123033&btnI=1&hl=en
Step 4 URL
https://scholar.google.com/scholar_alerts?view_op=create_alert_options&hl=en&alert_params=hl%3Den%26as_sdt%3D2005%26cites%3D17056599553763497017%26scipsc%3D
Final Alert URL
https://scholar.google.com/scholar_alerts?view_op=create_alert_options&hl=en&alert_params=hl%3Den%26as_sdt%3D2005%26cites%3D3568708134260123033%26scipsc%3D