For the beginning download and install Google Fit on your smartphone.
Open Data in the Welltory app
Select the required category, for example, Activity
Scroll down or tap Sources
Tap icon of the app you need. If the source requires integration with Google Fit, Connect through: Google Fit will be indicated
If Google Fit isn't connected yet, you will be prompted to connect it.
Choose your Google account and allow access.
Go to source app's Settings and enable data transfer to Google Fit