This is the best summary of all aspects tuber I have found. Not just for splitting but general info for size, health and condition.

https://summerdreamsfarm.com/dahlia-tuber-and-splitting-guide

I joined a few local dahlia Facebook groups, links are on the resources page, and got some first hand tips from a lovely grower in South Surrey, Karen, who I purchased most of my tubers from last year. Starting with healthy well draining soil and waiting until the weather is warm enough to plant in your local area is the key to success with dahlias. I amended my soil with manure, compost and wood ash, and added to each planting hole a small handful of a mixture of osmocote, bone meal and alfalfa powder. Dahlias are heavy feeders and a good shot of nitrogen at the start really helps. I planted 4-6 inches deep and spaced mine approximately 15 inches apart but every garden space will be different Dahlias should be planted on their side with the eye facing up. If there is no eye visible at the time of planting, you can just place the tuber in the ground lying flat and the eye will find its way to the surface. The most important lesson I learned was not to plant a shorter variety between 2 taller ones, it will get shaded out affecting both bloom and tuber production. I will plant the shorter ones in pots or in the front of a bed next season. When using pots I would suggest at least 5 gallons per tuber or small clump, they are generally big plants and need the room.

One trick to getting earlier blooms is to start the tubers off inside, in small pots, grow bags or even Ziplocs, a few weeks before last frost. You'll need to have the space and good natural light or grow lights so they don't get leggy. I did a test last year with the pairs of tubers I got. I started one of each in my sunroom at the beginning of April and when it was time to plant out in mid May I planted them side by side and without fail the pre-started tuber bloomed a couple weeks ahead of the other.

Most dahlias need to be staked as they get very tall and the blooms can get heavy. It's a good idea to add some staking when you plant so that you don't pierce the tubers if you add it later. There are lots of different methods for staking strategies, you can you google and see what might work best for your garden.

Regardless whether you plant your tubers in pots or in the ground do not water them until you see the new growth emerging. Dahlia tubers are very susceptible to rot until they have a root system to absorb moisture. This is another reason not to plant out until the soil temps are warm enough to trigger growth. The general rule of thumb is after all risk of frost is past and nighttime temps stay above 10 degrees.

Here is a link to Floret Farm planting guide and a video demonstration. They are in Washington State so have a similar climate.

https://www.floretflowers.com/resources/how-to-grow-dahlias/

https://www.youtube.com/watch?v=tHkEJ8SKviQ

After your tuber have sprouted and are happily growing, and hopefully not being eaten by slugs, the next step is pinching to promote more blooms.

Pinching:

When your dahlia reaches about 8"-12" in height or has 3-4 sets of leaves it is important to pinch them off to create more blooming branches. If you don't pinch you will get a taller plant but with only a few main blooming stems.

Here is a link to the 2 pinching tutorial videos I found most helpful. There are many more video tutorials to be found.

https://www.youtube.com/watch?v=cgiNb_IhGdc

https://www.youtube.com/watch?v=eGAVaqUIDbE

Fertilizing through the season:

If you planted your tubers with a shot of nitrogen to get them going it is a good idea to start regular fertilizing after they have been growing a month or so. I watered mine with some Epson salts at about 6 weeks in to help the growing roots better absorb nitrogen. I sprinkled slow release granular fertilizer around the base of each plant and and then started with a biweekly addition of Miracle Grow in, because I had some on hand but there are lots of choices for fertilizing. The first 2 applications were with the balanced 20-20-20 and them the bloom boost 15-30-15 until September. It's a good idea to lay off the bloom booster near the end of the season if you want to help tuber production.

Pest control:

Slugs - OMG the f'ing slugs were a nightmare here. They can decimate a young plant overnight so please think about protection for at least the first few weeks. I got some tall plastic cloche cone things and they worked pretty well. I was out nearly every night on slug patrol and putting out beer traps. I got some copper tape but call BS on that as I watched one crawl right over it.

Earwigs - I am not sure why I didn't know this after decades of gardening but earwigs eat plants, especially new leaves and petals. If you see your flowers petal tips look chewed or there is a lacy pattern to the chewed leaves it is likely earwigs.

Mites - If you get them spider mites are relatively easy to control with pesticide application as needed, there are many organic options available. Broad mites are another matter and they were a constant battle for me on a few varieties. They are so tiny you can only see them with a magnifying glass, they look like clear tiny mites and they move quickly. As they chew the underside of the leaf it curls tightly and so no matter what you apply it never quite gets all the way into the infestation. I took to stripping leaves and and cutting back so some plants looked a bit wretched for a while. Flower production was unaffected at least. I am going to try to get ahead of them next season by introducing predator mites and more ladybugs.

You should cut dahlias deeply to encourage longer stems, more blooms, and healthier plants. I found this out the hard way when I was too scared to cut deeply because I didn't want to lose an unopened bud along with the flower I was cutting. If you just snip off the flower you want then it will result in shorter, weaker stems going forward. The more you cut deeply the more new shoots and flowers are created, just like when you pinched. I really like to enjoy them in my garden as well as to use for bouquets so the ones I plan to cut lots of I will plant 2 or 3 of each variety.

• When to cut Harvest dahlias in the early morning or late evening when the flowers are hydrated. Cut when the flowers are about two-thirds of the way open, while the back petals are still stiff.

• How to cut Use clean garden snips to cut 14–18 inches below the bloom, just above a leaf node. This deep cut will encourage the plant to produce more side-shoots and tubers.

• Benefits Deep cutting will make your plants healthier and produce longer, stronger stems. You can expect to have a consistent supply of long stems throughout the season.

• Vase life To extend the vase life of your dahlias, you can place the stems in a sanitized bucket of water with a packet of Floral Life powder. You can also dip the stems in hot water and let them cool for at least an hour to rehydrate and condition them.

• Variety Ball, pom, and decorative varieties have a better vase life of about five days. Dinner-plate varieties are best used as an event flower or to enjoy for one to three days

Her are some tutorial videos:

https://www.youtube.com/watch?v=sZ5XcCYnzRs

https://www.youtube.com/watch?v=Dkgf5B3r-HU

https://www.youtube.com/watch?v=JA3dO3eggX0

https://www.youtube.com/watch?v=Aumghf3cHXE

Coming in summer.