You'll first want to make sure you're viewing the New Navigation Experience. If you're unsure, your left hand menu should look like the following with orange icons.
If your view doesn't look like the above, please follow this guide
Once you've confirmed you're using the new navigation you'll need to make sure you have an existing lead for that customer. Follow the below guide to find out how to create a new Lead.
Next, you'll want to open an estimate and go to the Estimate Details. Here you'll find an area for the Lead Details.
1. Click Link to a Lead.
2. Once you click Link to a Lead you'll be presented with a new screen on the left. Select the existing lead you want to link
3. Click to confirm the link