I prefer to cut the leads before I solder them - this way there's no risk of unsettling the contact while you cut the soldered lead, so this gives you nice solid reliable contacts. This requires you to hold the component with one finger while you're soldering it (otherwise it may fall off), which may get tricky as the component becomes pretty hot, but with enough kung-fu you'll be able to do it easily (e.g., hold it with the fingernail). I soldered thousands of components this way.

Another tip: when in doubt, put a little rosin on the pad and lead before soldering them, even though the alloy is supposed to contain rosin already. Again, this reduces the risk of a poor contact.

Never use cheap unstabilized soldering irons - those are only for rough jobs like soldering the lid on a tin can, and then just use the biggest iron you can find. For electronics, only use stabilized (regulated) irons (soldering stations), that allow you to finely tweak the temperature. Working at the right temperature is crucial; too hot and you burn components and oxidize pads before they get soldered; too low and again you may burn components because you're fiddling with the iron too long; or you make weak unreliable contacts. You'll figure it out with experience.

I actually prefer to use a soldering gun instead of an iron for anything except very sensitive multi-pin components that don't like EM pulses. Again, it's a bit of kung-fu to know when to push the trigger and when to release it; essentially, your brain becomes the thermal stabilizer. :) You need to "wax on, wax off" many hours before you obtain the skills, but then it's awesome. The gun allows you to do neat tricks like transport large drops of solid rosin, because you heat it up or cool it down instantly, as needed; a station, OTOH, is always at the same temperature.

Heating up both lead and pad at the same time is crucial. It bears repeating - crucial. Push the iron or gun against both parts. Don't push too hard, but make sure the thermal contact is good and firm.

Whether you wait 1 second before touching the heated pad+lead with the alloy, or you don't wait, depends on the technique. Whether you wait 1 more second before you retract the iron, or you don't wait, again depends on the technique. But once the liquid alloy has spread out, pull the iron at once, don't tarry. Then blow air over the hot area.

Nail clippers work just fine for cutting leads.

Sometimes you don't want to solder components too close to the PCB. This is true for those resistors, transistors, etc. that dissipate a lot of heat while working. Give them some breathing room. In general, don't push 3-pin components (such as transistors) too close to the PCB; when they begin to resist, you need to stop pushing. But soldering them might be tricky if they're not sitting stable in that position.

All that stuff about toxic metals and rosin fumes - I should probably be a brain dead zombie by now, I never washed my hands afterwards. Rosin smells kind of nice actually, similar to incense, but breathing the smoke directly is not pleasant; I doubt the smoke is more toxic than any other kind of smoke.

Finally: 2 hands are sometimes not enough. It helps if you're an octopus.