Each online shopping portal will have its clear return policy. Please read the entire literature and be familiar with the terms and conditions specified therein, or refer whenever needed.
From my own experience, I give below the general points in returning an item bought online.
1. Accept the item only if the packing is not tampered with. Please inspect the packet in front of the delivery person and only after ensuring that it is not tampered you accept and sign.
2. While opening the packet, if possible take full video. That will come to your help in case a clam is to be lodged later.
3. Test use the item immediately taking care not to break the warranty seals.
4.If the item is not as per your order, and you are not going to accept it that way, immediately send a communication to the online portal adding a photo of the item you received and the picture in the order. Inform them that you want to return the item and need a replacement or refund.
5. If the item is not functional, or defective or damaged, then also do as in 4, informing the reason. The earlier the communication, the better for you. You should not do anything on the item further, but keep it back in the same pack.
6. Good and established portals will communicate back to you and tell you the formalities once again for return and refund. They may ask you to file the standard format of return and refund/replacement in their standard formats online. Do that.
7. Good portals have their own arrangement of taking back(reverse pick-up) of the returned item. The authorised couriers will come to you and take delivery of the returned item. For that you have to pack it as suggested by the portal or courier. The online shopping portal will guide you in this. If not, you have to contact them, follow-up and get clear guidelines.
8. Get and keep records of return message, return despatch etc.
I had a couple of instances of return of the item and replacement and refund also. I did not have any problem. In my case, I also informed the seller also about return simultaneously when I informed the portal. That can make things faster.