1. This can be done by clicking on the "Flows" tab and then selecting "Create Flow". Choose the "Welcome Series" template and give it a name.

2. Set up the trigger for the flow. To trigger the welcome series when a new subscriber enters their details in a pop-up, you'll need to use a "Signup Form Submitted" trigger. Click on the "Trigger" step in the flow editor and select "Signup Form Submitted". Choose the pop-up form that you want to use as the trigger.

3. Set up the email series. Once you've set up the trigger, you can start creating your welcome series emails. You can add new emails by clicking on the "+" button in the flow editor. Make sure to include a delay between each email to give your subscribers time to engage with each email.

4. Test and activate the flow. Once you've set up the emails, test the flow to make sure everything is working correctly. When you're ready, activate the flow to start sending welcome series emails to new subscribers who enter their details in the pop-up form.

Alternatively, you could manually add new subscribers who sign up through the pop-up form to the welcome series flow. Another option is to customize the pop-up form with a checkbox that subscribers can select to opt into the welcome series, and then use a "Checkbox is Checked" trigger to trigger the flow.