When to Plant
After threat of frost—usually sometime between early September to early December. Dahlia tubers are vulnerable to rotting in wet cold weather so if in doubt or if there's particularly wet and/or cold spell in the forecast, it's best to hold back planting for another week or two. If you can keep their beds dry (plastic cloches/tunnels) you will have more luck pushing for an earlier planting. You can also pot them up in moist (not wet) potting mix, in a sunny dry spot to give them a head start before planting out - though it is a fine art to keeping them dry enough without being too dry in pots.
Where to Plant
Dahlias need a nice sunny spot. Make sure the soil has fantastic drainage, dahlias hate staying wet—if you're worried your soil might not quite drain as well as it could, plant on a slight mound / raised bed.
Ideal spacing can vary between varieties but in general somewhere from 30–50cm is a good average. Closer spacing works best if you plan to dig & divide yearly, and larger spacing works best if you plan to perennialise your dahlias and dig & divide much less often (2–3 years).
I plant my beds for cutting/tuber production 35cm apart in the row, and about 40–50cm between rows.
Plant tubers sideways (or on a slight angle down with the eye being raised slightly) 5cm below the soil level, you can fertilize with general/bulb/potato fertilizer now, or side-dress after shoots appear. I like to add my compost to the top of beds (rather than mixing in) as it's easier, and helps act as a weed mulch, a good couple of centimeters is plenty, and take this into account when setting your planting depth.
To have your plants producing the most flowers for both your garden and cutting, it makes a huge difference to pinch out (by hand or snips) the growing tips once they reach about 20–30cm tall, taking it back by about half (if you can muster up the courage I promise it's worth it!).
This creates a nice deeply branched plant, and the deeper you cut the better the
stem length for cutting. I like to get my pinching done just before I set up my support system.
Dahlias really benefit from some sort of support system to hold them up once they start getting above knee height. There's so many different methods depending on your scale and individual preference.
For planting in my home garden, if there wasn't a nice nook in between shrubs to do the supporting for me, I individually stake (bamboo/wooden) each plant making sure to set it up when planting so as not to damage the tubers later.
For larger cutting gardens you could net (Hortonova netting from Polynet is a great option), using waratahs or strong wooden stakes to hold the netting tight. Netting does provide the most support, but it can be quite the nuisance for harvesting through & is the least fun system to clean up your rows in Autumn. If you are in a high-wind zone I think the benefit would make these nuisances worth it though.
An alternative to netting is instead opting for a simple corralling system (waratahs/rebar/wooden stakes & baling twine).
Dahlias appreciate a bit of love throughout the season, once established they like regular deep waterings (mulches help maintain moisture if you're trying to conserve water).
After they start flushing with blooms, regular deadheading (removing spent flowers) really does make a world of difference in keeping them flowering
If you want to spoil your plants, dahlias also appreciate regular foliar feeding,
and/or occasional sidedressing of fertilizer, especially after peak flushes.
Dahlias are best cut in the early morning or cool of the evening, straight into clean cool water. If for local flower sales, conditioning in a chiller is highly recommended. Don't be afraid to harvest deep (long stems), the harder you cut dahlias the better and stronger stemmed the next flush will be. Dahlias don't open too much after cutting so cut when about 2/3 to 3/4 open (or before pollen sheds with singles/open centered varieties). Make sure you keep the vase water changed regularly, and kept away from direct sun, heat, & the fruit bowl for the best vase-life.
Dahlias can be left in the ground for about 2–3 years before they'll need lifting & dividing. They can however be susceptible to rotting over winter if they don't have amazing drainage or your ground freezes. To play it safe you can lift in late autumn and store somewhere dry, cool & out of the sun, and replant after dividing in spring.