Hi Ralph, soldering always seems difficult, and sometimes it can be without the right tools, but it's much easier to dismantle a badly soldered model than it is a badly glued one! I offer my experience as a purveyor of Carr's solders and fluxes, but there are several other sources of solder which are much the same stuff, just with different names.
As someone mentioned above, start with the largest items first, usually chassis and boiler/body shell, using a higher temp solder such as 227c/440F tin/copper and 221c/430F tin/silver. Having silver in the recipe makes a solder flow very easily and 'wets' metals faster than solders which do not contain silver. Sometimes I pre-tin parts with silver-bearing solder simply because it makes whatever other type of solder I am wanting to use, stick much easier/faster - this is important because if you take too long to make a joint, the whole part is heating up and you risk other joints coming apart. By the way, if you have any 221C/430F tin/silver, it is great for tinning the tip of your iron, to protect it from corrosion and improve heat transfer.
Silver-bearing solders spread out very thin, making them great for laminating flat parts together, while solders without silver, such as the 227c/440F tin/copper tend to build up thicker, making them good for joining parts at right angles because you can build up a thick fillet in the corners for extra strength.
The next step down in melting points are 60/40 tin/lead (183C/356F) and 179C/354F tin/silver/lead, again the presence of silver makes the latter quite runny when molten. By this time you would be using these attach smaller details. I find that I often don't need to use lower-temp solders than these (except when soldering white-metal (pewter) castings, which have a similar melting point, around 221C/430F) because by now you would be using only a tiny amount of solder on a pencil-tip iron so you can get in and out before anything nearby has a chance to come loose.
As an aside, 179C/354F tin/lead/silver is very handy for soldering electrical wiring, and I find it very good for soldering nickel-silver rail to copperclad sleepers because it does not leave any blobs to be scraped off.
Below that comes 138C/280F and 100C/212F bismuth/lead solders which may not be particularly relevant to you but I mention them for the sake of completeness - these are used in the UK/Australia/New Zealand where we often see cast white-metal details, whereas these seem to be comparatively rare in the USA.
Other small tricks to help the job along....
It is often useful to use pins or screws for alignment purposes, which can be cut off and filed flush later. I prefer to pre-tin both mating faces separately, then press together, dab on liquid flux with a small artists' brush then apply heat, so that you are never left wondering if the solder has penetrated the joint properly. While some people prefer to heat a joint and feed in solder wire, my hands are not steady enough so I put the solder on the tip of my iron and then run the tip along the joint which I have already liquid-fluxed. None of the Carr's solders are flux-cored, so you must apply a liquid flux of some sort....the advantage of this is, the solder generally only goes where you put the flux, so a bit of care with how you go about it will greatly reduce clean-up later. I am always impatient to get a model finished, and I view scraping off excess solder as a colossal waste of time. And another trick for reducing the amount of clean-up due to solder spreading where you don't want it: hold the parts together where you want them to go, and rub all around with a soft pencil - the graphite stops the solder from sticking so it won't spread out beyond the joint.
Hope this is of some assistance,
Regards
Paul Woods
Whangarei, NZ.
