its soo easy :-)
open your Instagram home page and select option "search" like this
now just click on search bar for further options like this
now you got multiple options to search about.
for example: Top , People , Tags , Places.
select option "Tags" LIKE THIS
now write name of hashtage in search bar like this
click on hashtage option like this
now a new page will open up with desired hashtage to follow up.
click on option "follow" like this
now you are following your desired hashtage.
you will see all incoming posts in news feed.