How to create product variants? How to add dropdowns for selectable options like size, color etc.? How to create user input fields for my products where user can enter some details that I require for the product (like t-shirt slogan, software machine code etc.)?

Variants which affect price (dropdowns only)
For products like t-shirts, shoes etc. for which each variant has a different SKU number and you want to maintain a different stock for each variant and even want a different price for each, you should check the "Variants having individual SKU/Price/weight/stock" checkbox while adding or editing a product in E-junkie admin. The price field will automatically get disabled as your product variants will each have their individual price.

Press "SUBMIT" button and in the next screen you can create upto 3 option names and then enter the required values of the product as specified on that page. Press SUBMIT or NEXT till you reach the last screen and there you will get the code with the dropdowns included.

Variants which provide you more information about the product (dropdowns and textfields)
Sometimes all your variants might have the same price and you maybe not interested in maintaining an inventory OR sometimes, you want user to enter data (like a t-shirt slogan) rather than selecting options from a dropdown. In that case, check the "Variants which provide more information" checkbox and press SUBMIT.

In the next screen, you can enter upto 3 option names and then provide values (in case you want a dropdown) or leave blank the respective option value fields below based on whether you want to create dropdowns or textfields. Press SUBMIT or NEXT till you reach the last screen and there you will get the code with the dropdowns and textfields included.

Note:

If you want, you can check both the variant options and use them together. That might be useful if you want 6 dropdowns or maybe if you want to maintain inventory of your t-shirts which differ in size and color but also want the buyer to enter a custom slogan.