I have made hundreds of single and double flares with the tool shown and even with one which included rollers and tho they worked I was never happy with them.
3 years ago I purchased an Eastwood flaring tool, there are others too probably by the same manufacturer, and could never be happier. It handles up to 3/8" tubing and every end is perfect with 2 pulls of the handle and a click of the tool head. It can also put a "bulb" end with a little practice. Yes it was over $100 and since my son has done his complete brake and fuel system and I have done a rear end change and disc brakes I can say it was worth every penny. I'd show a photo but it's at his garage right now and are ez to find on the Internet.
Over the years learning to do things on my own and purchasing good quality tools that will last a guy time me a life time has not always been possible but was done as the need arised. I don't a shop or make any profit but sure am happy when thing go right because the correct tool was there to use.
We started using nicopp and so far been pleased.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Is that copper or nicopp?

Doing a test:

Now put the OH CRAP flare nut on the tube.  

Please remember the flare nut MUST be on the tube BEFORE you do the flare if there are any bends or the other end has already been flared.

Ask me how I know this Lol

Great Tutorial TED   as always

TUTORIAL  FOR  DOUBLE  FLARING  TUBING
 
Remove burrs from inside of cut end of tubing
  
 
Here’s the double flaring tool as a kit for the various tubing sizes
  
 
Select the appropriate adapter.  In your particular instance, use the one labeled ‘3/8’
  
 
Install the tubing end in the flaring bar so the end of it protrudes to the level of the first ledge on the adapter.  Tighten the bar so the tube does not slip.
  
 
Put oil or some kind of lubricant on both the stem side of the adapter and at the end of the tube in the flaring bar.
  
 
Place the adapter with the stem inside the tube.
  
 
Center the flaring yoke over the adapter.
  
 
Turn the feed screw on the yoke until the adapter is flush with the surface of the flaring bar.
  
 
Remove the yoke and examine the partially flared end of the tubing  to insure there is no splitting occurring and it's even all the way around.
  
 
Reinstall the flaring yoke and center it over the end of the tubing.
   
  
Again turn the yoke feed screw until it as tight as you can get it doing it by hand.
  
 
Remove the flaring yoke and examine the completed flare.  If it looks like it does not have a surface that’s flattened out enough, reinstall the yoke again and screw it down tighter.
 
 
Remove the completed tubing from the flaring bar and examine it for any defects.  At this point it is expected to good to go.
 

 

Lorena, Texas (South of Waco)

The size designation for the fitting that goes into the Summit fuel line is ‘3/8” Inverted Flare’.  A normal straight threaded fitting will not work as it does need a bevel on the fitting to match the bevel within the fuel line so insure a positive seal.

Lorena, Texas (South of Waco)

thanks for the comments, an illustration never hurts. A question, how big is the thread of the image connection?

I’ll have to disagree with Charlie regarding the use of a single flare end on the fuel line.  And I also have an issue with using copper instead of steel for fuel lines.  I’ve been both ways early on (teenager) and the single flare ends to have a tendency to split at the flare opening while copper is seamed tubing which also has a tendency to crack along the seams when vibration is present.
 
A fire as a result of a broken or cracked line would not make for a happy day.  My recommendation is a steel line with the appropriate double flare.  The quality of the flaring tool also makes a difference.  If you need some kind of pictorial showing how to do double flares, just let me know and I’ll see what I can work up.

Lorena, Texas (South of Waco)

A single flare is ok for fuel line.


Lawrenceville, GA

with this connection is the one with which I am not very convinced.


The one on the left is the original flare that is double and I could only make it simple that you see on the left.
I am also looking at another option.

I am also looking at another option.