The process to enable the app in the theme is very easy. You can directly enable the app from the theme customization.
Embed in Themes (Any Theme)
From the Online Store, go to the customize section of Theme.
Inside the customize option, click on Theme settings. A side menu drawer would open where you can find the option to Embed the apps into the theme. Enable the embed and click on save.
Later you can configure one of the products from your store and check the product page to see whether the customization fields are showing correctly on the page.
Theme Editor (Shopify 2.0)
If the placement is wrong, or you want to change the position of our app field, then you can follow the instruction below. This is only available for the theme that supports Shopify 2.0.
First, from the customization window, select Products, and then select the correct product template you want to edit. You will find only the default template from the theme if you do not have multiple templates. Select the default template in that case. From the product template, change the product preview to the product configured with our app. Next, inside the Product Information section, add our app block. Later you can place this block into the correct position and click the save button.