Nick Saporito, the author of this video, states at the start that GIMP w0uld be a better choice for making a T-shirt mockup but he says you can use Inkscape.
See my post, GIMP For T-Shirts, July4, 2017 for making T shirts in GIMP for Amazon Merch.
By Nick Saporito:
Published on Sep 27, 2016
Design a Shirt with Curved Text and Prepare for Printing – Inkscape Tutorial
Published on Oct 7, 2016