I feel for you. Soldering takes a touch. Practice on parts like what you are going to do.
I found, if I laid solder on each part to be joined, I could sneak up on it. I didn't know of cold fastening, or blind rivets, so I soldered my underrib on. I laid flux on the barrel flat, and started heating up the barrel. I put a dowel in the bore, and laid the barrel on a 2 X 6. This did seem to help. I got the barrel to temp, and the solder flowed. I used lead free solder. Mostly tin actually. I laid the flux only on the part to be soldered. I then did the corresponding flat on the underrib. When cool, I then filed the parts I did not want any solder on. I then got a steel, or conduit pipe that fit the ramrod contour. I got 4 C clamps, and with the solder filed down, I laid the piece in place, on a line of flux. I tightened the clamps, making sure it was straight, and without uneven spots.
Now, I heated the entire piece. I also used an hardware torch. In short time, the solder started flowing. I added solder to fill the voids. When done, and browned, it looked real good. I filed and sanded any drips or overflow.
Flux can burn. This negates its effectiveness. The 2 X 6 acted as a bottom flame, helping heat the piece. The tendancy is to heat directly where the solder is to go, but in reality, heat around where it is to go. The solder will flow to the heat. You can wick it along with the heat as long as you do not burn the flux. Solder will not flow without flux. Neither silver, nor gold, nor lead/tin.
The big thing is to keep it clean. If it burns, cool it down, sand it off, and start again. I like to cut small pieces of solder and lay it on the flat. When it melts into the flat, and flows, you have the heat. Using borax, or boric acid will be too hot for the solder. Good for silver or gold, but not lead or tin or an alloy.
The big thing, is to practice first on something simliar to the part to be soldered. Get used to how the heat affects the solder and metal. Watch how the flux burns and when.
When I teach my students, I have them melt copper wire. I prefer 16ga. I just like to get them used to the way the metal is affected by heating. I then have them sprinkle borax on the melt. I then have them fuse the wires. They do "T" joints, butt joints, and lap joints. No solder, just fusing. Later I have them silver solder the wires.
You will need to find a way to practice the same thing with what you are doing. It will seem impossible untill you get the feel for it. It will fight everything you do. Then one day, it will just flow together and you will be relaxed.
Watch the heat, don't burn the flux, and take your time.
It will happen.