Ed, I wouldn't attempt to do something like this with a hand soldering iron, I'm fortunate in having the loan of a reflow oven and a hot-air workstation.
Step 1: Clean the PCB
Step 2: Start with the side of the PCB with the most/smallest parts and add solder paste to all the pads. I do this by hand under a USB microscope using one of these gadgets to get a lot more control than the syringe alone https://www.vtech-smt.co.uk/jbc-dpm-a-s ... -dispenser
. I use this solder paste, https://www.mouser.co.uk/ProductDetail/ ... PJPg%3D%3D
- it isn't too viscous, has a fairly low melting point and keeps more or less indefinetely (a lot of solder pastes 'go off'). The trick is to apply just a tiny amount of solder paste and I use a blue nozzle on the syringe as the best compromise between precision and not needing too much pressure to push the paste out.
Step 3: Place all the parts onto the pads with fine tweezers, gently pushing the parts down onto the paste. If a part is slightly askew I adjust it with a long pointed tool.
Step 4: Place it into the reflow oven, select the right heating profile and press start.
Step 5: Remove the cooled board from the reflow oven and inspect. Fix any solder bridges with wick and an ron.
The RTZ DAC PCB also has parts on the underside of the board as well. For that I just replace step 4 with the use of my hot-air workstation - that keeps a lot of the heat from the topside. I've had no problems with parts dropping off the topside.
If I do need to had solder a part, such as a jumper resistor, I paste the pads, put the pad in place and hold it in place with a matchstick while I touch a soldering iron to the part/pad.
All of it took a lot of time for the first boards but I'm steadily getting quicker, though I don't plan to make too much of a hobby of it!
I'm sure someone will have an observation or two but what I've descibed works for me!