Pin it through the buttonhole on your lapel.
You can let the needle poke back through the front of the lapel or keep it hidden underneath.
The fastener on the end holds the stem in place.
The reasons depend on the occasion and the person.
Sometimes for symbolism, like flags or poppies. Sometimes for tradition and uniformity, for example at weddings. And sometimes just to add extra flair to their outfit.
All lapel accessories should go on your left hand side.
Some claim it’s to place the pin close to the heart, but the real reason is less romantic.
Originally you were able to button lapels, just like the rest of the buttons in your jacket, to keep the top of your chest warm. So the buttonhole had a practical use. On menswear, buttonholes are always on the left hand side; this makes them easier to fasten yourself (for right-handers).
This is also why women’s buttonholes are on the right hand side, as the expectation was that someone else would fasten them.
Absolutely. The same rules apply as with standard jackets and blazers.
Note that with a tuxedo, you’ll want to stick to a white flower for an elegant look.
On a sweater, you’ll need to pin through the fabric, so make sure you don’t pierce too many holes.
There is no hard rule. The fastener at the end can poke through the front of the lapel or not - it’s up to you depending on what you think looks better.
It really boils down to what type of wedding you're attending. But unless it's an extremely fancy wedding, yes.