Development workflow
A Shopify store has multiple themes within it. The theme that has been set as the Current theme should be in sync with the 'main' branch in the github repository for that project. The main branch is usually called dev, development, or master (check with lead developer for project).
Duplicate the Current theme and rename it to something like [Theme] - TWP [Name] eg. Icon - TWP Marc
Using github desktop, clone the github project to your machine. Follow the readme instructions
Create a config.yml file that links to your duplicated theme.
The password can be found via the Shopify admin area in Apps > Manage private apps > theme gem (it may not always be called theme gem)
The theme id can be found by navigating to Online store > Themes then clicking Customize on your duplicated theme. The url will have the theme id eg. https://store.myshopify.com/admin/themes/[THEME-ID]/editor
See theme kit for more info
Create a branch for whatever feature or bug you're working eg. product-page-accordion
Most projects will be running Shopify's slate tools so you should be able to run slate watch in the console to sync your local dev files to the theme we duplicated earlier
Commit as often as you can in case you ever need to roll back changes you've made, and comment appropriately
When the work has been completed and ready to test, create a pull request and notify the lead developer so they can approve the work