My flaring tools and union nuts are from
Set the union on the pipe before you start flaring.  You cannot put the union fittings on the pipe once  the the pipe flare has been set.  I learned this the hard way.

  If the pipe has any irregularities in its circumference because it may have been flexed, the nuts may be difficult to move.  It is possible to move them, but with two wrenches, Take one 10mm wrench, and one 9mm wrench.  Use the 10mm to turn the union  back and forth.  Use the 9mm wrench to apply  pressure in the direction you want the union to  go. You rest the 9mm wrench on the 'shoulders' of the union to apply pressure.   Note that these are the union nuts from Pleiades.  They are steel, so the head won't get rounded, and ALL the heads (for both 3,5 and 4,5mm pipe)  are 10mm - no more poking about for that odd 8 or 9 mm wrench!

 In this case, I am pressing the union away from the end of the HP line.  Or, I could be doing shadow puppets of a drooling snake smoking a cigarette.                      

Here is the 4.5mm flaring tool.

Set the HP line in the flaring tool such that the end of the line is even with the groove cut into the tool.

Tighten the two closure bolts on the side of the tool.  Make sure it is tight, because you do not want the HP line to move inside the tool when you create the flare.

The end of the HP line visible near the end of the tool.

Inserting the threaded part of the flaring tool.  The camera focused on the wrench, not the tool, but you can get the idea.

Tightening the flaring tool.  A deep socket ratchet is easiest.  A vise is very helpful.  The nice thing about these small flaring tools is that they can be used on pipes on the car.  In the last photo I am holding the tool with some pliers and the ratchet. 

Making the flares takes some practice.  You will have to develop a sense of how much pressure is necessary to get the correct shape on the line.   The resistance is minimal at first, then gradually increases and then steps up markedly.  After the big step-up in resistance, it still takes a few turns to create the flare.  Plan to do some practice flares.  Also be prepared to have tightened the tool too tight, which jams the HP line into the tool.  Then you have to pry the pipe out.The flared pipe.  Again, sorry for the bad focus.