Open Klaviyo and click on Audience.

Click on Lists and Segments

Click on Create lists/ Segments button.

Click on Segment

Write a name for this segment.

In the Definition section, click on the Select a condition field.

From the menu, select What someone has done (or not done).

Click on Choose a metric field, scroll down to Shopify metrics, and select Ordered Product. This is how it should turn out so far:
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Click on Add Filter.

Click on Choose property field, and select Name.

In the Equals field, search for the product that you are offering a subscription selling plan for it and select it. This is the final segment:
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Click on Create Segment button.

From Klaviyo, click on Campaigns.

Click on Create Campaign.

Select Email and click on Create Campaign.

Select a name for this email campaign.

In the Recipients section, Send to field, select the segment that you have created.

Click on Continue to content.

Fill in the required field and click on Drag and Drop to design your email(it's the most convenient way to create your emails)

Select a template and click on Use this template.

Edit email content and send a test email to yourself by clicking on Preview and test.

Click on Save and Exit.

Click on Continue to review.

Click on Schedule or Send.