Login to your store's administrative panel and navigate to Sales channels > Online Store > Current theme > Customize (green button in screenshot).

Once the customization screen loads, you will see a menu button with three dots "..." at the top left, click it and go to Edit code.

Most themes will have a single layout file (in this case, theme.liquid) which you will need to modify. If more than one file controls the look and feel of your website, make sure you repeat this step for each file.

Edit your theme file and scroll to the bottom until you see a </body> tag.

Once you find the body tag, insert the Mantis pixel above it (in this case, between lines 127 and 128) so that the <script> tag is above </body> like so (be sure to replace XXXXX with your advertiser id):

After you added the code, be sure to hit the green Save button that appears at the top right.

For the last step, you need to add the pixel again to your checkout process since this is controlled by Shopify and does not use your theme. To do so, go back to your administrative home page and locate the Settings option at the bottom left.

In the settings popup, navigate to Checkout and accounts and scroll down until you see Order status page.

Paste the same script you did in Step 1 into the Additional scripts box then hit the green Save button that appears.