I'm also going to open with a disclaimer that I've not got/built a Quantum, but I have build an ET WiFi switch and assembled hundreds (no exaggeration) of boards of similar complexity. As has been said above, SMT assembly work goes much easier with practice but the component sizes that ET uses are definitely within the realm of what a first-timer can handle.
I'll share some things that I've learned about assembling boards like this. Like with everything in this hobby this isn't the only way to get to the "end game" but it does work for me. Have a read, think about what makes sense to you, try it out and keep asking questions to fill in the blanks.
Some basics:
1.) Don't use a 40W iron, the 15W iron is what you want for this job.
2.) If your tip is oxidized, which it sounds like it is, clean it with a damp sponge,
never with sandpaper. Soldering iron tips are typically plated with an a oxide resistant layer, which sanding will go through quickly. Once the plated layer on the tip is gone the rest of the tip will erode away from the heat and oxidation. Tips also last longer if you keep a good "blob" of solder on them when they're sitting in the stand between uses. (Wipe this away with the sponge before you go to solder a joint.)
3.) The type of iron tip you want is a personal choice, but I'd recommend staying
away from round and sharply pointed tips. My preference for this kind of work is a fine, flat chisel tip with or without rounded corners. One of the enablers to successful soldering is your ability to get heat from the iron into the leads and molten solder - this requires surface area on the tip of the iron which pointed tips do not have. As ttabbal said above "The idea is to heat both parts, the pad and the lead, equally, and get the solder to flow between them." I do most of my work like this with a tip that's about the size of the SMT resistor pads that are used on the Quantum. (Photo here shows my tip of choice for this kind of work on top of an ET WiFi Switch for size comparison.)
4.) Leaded solder (ie. Kester 245) and flux are your best friends for this kind of work. Leaded solder melts at a lower temp then lead-free solder so it's easier to get the heat that you need from the iron into the joint/parts. When you heat the solder with the iron the outermost layer will immediately begin to oxidize, and so too will the pads and leads that your trying to solder, and all of this inhibits the solder from flowing into a nice joint. Flux will clean those oxides away. "Rosin core solder" has some in it already, but I prefer to be able to add more to help clean up joints. You can get flux in many forms, I prefer
this one. It's messy, it's sticky, it's awful stuff, but it really makes a difference for jobs like this, and when you're done it cleans away with alcohol and a stiff-bristled brush. Don't use plumbing/acid flux on electronic assemblies though, that's asking for a future failure.
Now, to put parts to the board:
1.) If you're starting fresh, don't tin all the pads. If you want to do some pre-tinning, pick one pad from each component, leave the rest.
2.) Restrain your board in something so it won't move when you work, you'll need both hands free. (Clamp if you've got one, tape it to your work surface if you don't.)
3.) Work one part at a time, and I generally work from smallest to largest. Pre-tin one pad for the part you're going to install and build up a little bit of solder on that pad. Using a pair of tweezers if you can grab the part, or a stick on top of it if you' can't, hold the part in place as best you can over top of the pads. While doing that re-melt the solder on your tinned pad and let the part settle down flat onto the board. Take the iron away first, wait a few seconds for solder to solidify, then tweezers away second.
4.) Inspect your part to make sure that it's sitting properly on top of all the pads - a little overhang is OK. If you need to adjust the placement you've only got one joint to re-melt. Get in there again with the tweezers to support the part, add heat, adjust the placement, remove heat, remove tweezers. Don't worry if this solder joint isn't "pretty", you'll come back and clean it up later. But your part is now "anchored" to the board in the right place.
5.) If you're installing a large multi-legged part (like an opto) carefully tack the lead in the opposite corner. Inspect again that you're still sitting well enough on all the pads.
6.) Dab on some flux around the leads of your part. Don't worry if "too much" comes out, you're going to clean it away later. Melt a little blob of solder on the tip of your iron, touch it to each of the un-anchored leads by putting the iron into the "corner" that's formed between the component and the pad for larger parts. You can also put the iron down on top of the lead, pressing it in to the pad, for leaded parts like optos. Hold until the molten solder wicks in to the corners between the component and the pad, this usually happens really quickly so you don't need to hold for long. (Disclaimer: I've done my "NASA certified solder training" course and strictly speaking transferring solder like this is improper technique. When working at home just make sure you post-solder inspect to make sure you've got a good shiny solder joint.)
7.) When all the other leads are soldered down go back and clean up the "anchor" joint by adding flux and heat to re-flow the solder. More solder from the iron probably isn't required.
8.) Using alcohol and a short, stiff brush clean away all the flux residue.
Here's the ET WiFi switch and another board of my own design, both assembled with the above techniques.
Go forth and assemble!
