To create configurable products,

1. Go to the Magento 2 Admin Panel.
2. On the left navigation bar, click on Catalog and then on Products.
   
3. The Products page appears as shown below:
   
   The imported products along with their details are displayed in the grid.
4. To add any new simple product, click on Add Product button and fill in the required details.
   
5. Now you need to add the custom options for the simple products you had created as shown in the images below:
   
   Then click on Add Option under Customisable Options and add the required custom details.
   
6. To convert a simple product to configurable select that simple product you want to convert and then under Actions click on Custom to Configurable.
   
7. After clicking on Custom to Configurable, the product will be changed to Configurable product and the various attributes added by you will be reflected on the front end.