We're not talking about dubious 1970s fashion items here, but about the features of no particular value that railway engineers put on the tops of water tanks, both tender and locomotive. Depending on the prototype, the flares might be either flat or curved, and the corners of the tank might be sharp or curved. Generally, curved corners are more difficult to deal with because the flare has somehow to be wrapped around the corner.

On the prototype, the flares were usually separate componets riveted to the sides of the tank. Depending on how obvious that it, it might be necessary to reproduce that on the model, rather than make the flare an integral part of the tank sides that is then bent to shape.
Making the flare integral with the sides of the tank is a common kit solution. To wrap the flare around a curved corner, a series of cuts is made vertically through that part of the sheet material that will form the flare, creating a set of "fingers". The corner is made by curving the sheet around a rod of suitable diameter, the flares are bent, and then the fingers are splayed out in a sort of conical array. Then one fills the gaps between the fingers with solder or Milliput, and files it all up to create a smooth conical surface.
I've done that in the past, but never been entirely happy with the result. Somehow the result, instead of being a smooth cone, is faceted. I think it is the difference between the relatively hard nickel silver (or whatever) and the soft solder, which results in the latter being preferentially removed. And while the outside surface of the flared corner is not too bad, in that at least it can be filed with conventional tools, the inside surface is much more difficult to work on. And of course, at the usual viewing angle from above, it is the inside surface that is visible.

So I decided to try something else. The flare is made separate from the tank sides, cut in the flat in such a way that it can be curved at the right point around a suitable conical surface in order to form the corner. Once done, it is simply a matter of soldering the flare in place on the top of the tank. Download a data sheet here that shows how to work out the size and shape of the flat.

For flared tanks like the one above, I make the flare in two parts, each comprising one side and half the end. There is thus a join in the centre of the end, but in many cases there is a lamp bracket or something similar there that can be used to disguise the join. If not, it is necessary to make the best butt join that I can. But that is still easier than trying to make the whole thing in one piece, because getting the two corners in exactly the right place is next to impossible.
Flares
Nick Baines   Model Engineering
This tender has sharp corners, and the flares are clearly separate components, riveted to the sides and ends of the tank, and the model follows that. Here's how I did it.

The flares are curved in section, so the first thing is to make the curve. This device is to clamp and curve a sheet of nickel silver of width greater than the longest flare required. It comprises three bars (I used aluminium but steel is just as good). Two are identical and have dowels at each end, press fitted into one bar and a nice running fit in the other, to hold them squarely together. The third is screwed on to one of the two, leaving a step at the top.
The step is to locate a round bar of a suitable diameter. The sheet of nickel silver is clamped in place and the whole thing is held securely in the vice. The free end of the sheet is then bent into a curve.
The lower edge of the flare is marked out and cut to size.
Rivets are then impressed along the lower edge using my riveting tool.
The flares on the tender tank sides taper from rear to front, so the height was adjusted by careful cutting and filing.
The flares are made over-long, because the rear corners have to be bevelled at 45 degrees where the sides and end meet. To make the bevel accurately, I took a piece of round bar of the same radius as that of the flare, reduced it to half round, and cut off the ends accurately at 45 degrees. The finished length in this case was the same as the end of the tank, so that I could cut the flare for the end all in one go, but that isn't essential.
The final photo shows the flare clamped in place on the saw table. It is sawn roughly to size, and finised by filing up to the end of the bevel piece. Then everything should fit together with nicely mitred corners.