Use these instructions respectively(in the order already mentioned) :

1.First of all you need the latest version of "Instagram" official android App on your phone.

2.Install the "Easy Downloader" from here.

3.Go to Easy Downloader and then go to "settings" and make sure these options is checked on: "Full screen", "Download mode" and "Single folder".

4.Select the camera icon on the settings page of Easy Downloader. (You see it on the following picture) :

5.You will be redirected to your Instagram App. on the Instagram App go to the post that you want to download a picture or video.

6.Select the "…" icon (as you see on the following picture):

7. select the "Copy Share URL" option. If you tried to download a picture, now the picture downloaded but if you wanted to download a video you'll be redirected to Easy Downloader. select the "Floppy" icon to save the video(as you see on the following picture):

That's all, Enjoy it.