Among many things I have observed when trying to help some of my friends with their swimming is how difficult it is for them to correct their flutter kicks. Often times, the kick becomes more of a drag than a boost to their speed. More than just as a basic in swimming, it is skill that has to be continuously improved over time.
Browsing along to find advice, I found an article written for triatheltes in improving their swimming leg but can help normal folks like us improve on our swimming form. Took notes on some for myself as well. This is, by no means, written by me and all credit is due to the athlete who shared his insights for us to use.
Hope this helps.
Unstructured, raw, and partially edited 