There is no easy way to do this, practice is all you need. If you study flames and focus on this blade you could achieve it in 3-4 days. That is assuming you draw at least 25 a day.
This is known as brute force learning, the idea is that you take something you don't know and fail at making it around a 100 times. Artist use this when ever they get stuck with a single topic, like drawing a nose or eye, then just keep trying for a long time.
The important part is that you keep making the full sword, even if it is low quality, don't start a new one till it is done.
To make things easier you could take a image of a flame and a sword then place the flame over the sword blade. Use this as a background to draw over to get the sword you want.
Also this is a good set of "how to" tutorials on pixel art: https://imgur.com/gallery/tFxyX