Adding a gang sheet builder to a Shopify theme
Adding a builder to a store is twenty minutes of settings, not a theme project, as long as the app uses a theme app block rather than editing your theme's code. This is the order to do it in.
Theme app blocks, and why they matter
Shopify's Online Store 2.0 themes let an app add a block you place in the theme editor like any other section. The app never touches your theme's files, so a theme update cannot break it and uninstalling leaves nothing behind. If an app asks you to paste code into theme.liquid, that is the old way, and Shopify is retiring it.
Set the sheet before the button
Open the app's settings first. Set the sheet width to your film, the shortest and longest sheet you will print, the rounding step, the safe margin and your rate. A customer who opens a builder before these are set gets someone else's defaults, and the first order is a refund.
Place the block
In the theme editor, open the page you want the builder on. A dedicated page called Gang Sheets or Build Your Sheet, linked from the main menu, works better than a button buried on a product page, because customers search for it by name. Add the app block, set the button text and alignment, and save. The builder itself opens as its own page on your domain, so the theme does not have to hold it.
Print types
If you print more than one kind of transfer (standard DTF and UV DTF, say), each has its own width and rate. Set them up as print types and either let customers choose when they open the builder or point a second block at a specific type, so the UV page opens the UV builder.
Test an order end to end
Before you announce anything, build a sheet yourself as a customer, with a deliberately bad file (a JPEG with a white background) to see the print check, and buy it with a test gateway or a 100% discount code. Check three things on the order: the length and price match what the builder showed, the line carries the sheet's details, and the print file downloads at the right size. Then open the app's queue and mark it printed, so you have walked the whole path once.
With SheetGangster
SheetGangster ships as a theme app block with one button, and its Settings page holds everything above: width, length range, rounding, margin, rates, volume breaks, named sizes and print types. The order arrives in the print queue with the PNG and PDF, and the file link can also be written to the order for other apps to carry. The free plan is enough to run the test above.