Create site collection using custom site template SharePoint Online

1. simply proceed as you would normally to create a new Site Collection, however when it comes time to select a template to base the new site collection select the Custom tab (on the extreme right), which should only have one option – < Select template later … >.
2. Select this option and continue with the new site collection creation process.
3. Now navigate to the new site collection URL. You will see the Solution Gallery heading.
4. Click on the Solution Gallery, it will empty( for first time).
5. Now you need to upload your Custom Site template.
6. After Uploading, please make sure it is Activated if not click on the Activate button.
7. Once activated close the dialog and Now You will this template under Custom Tab
8. Select the Custom Site Template and Click Ok