Most of us start bringing the dahlias out in early to mid April, and then starting them under light. I have posted last year, with photos, what I usually do. When they have sprouted, I plant them up (after further separation if necessary) into pots that fit the tuber. In May, when the days are nice I put them outside during the day, and carry them back to the garage at night.

Here is a link that might be useful: splitting dahlias

In early April, I usually lay them out under lights as shown below:
{{gwi:631805}}

Then in a week or so, we start to get sprouts. Sometimes, it takes a lot longer than a week or so to sprout, to which many of us will attest.
{{gwi:631806}}
{{gwi:631807}}

Once they have sprouted, you can pot them up, and place in a bright spot. If putting them outdoors, you will have to acclimatize them to the stronger light, but they won't get as "leggy" as they will indoors in a window.

This whole process can be started in late April as well.

Patience is a virtue, triple_b, which we must all learn. Mother Nature won't be rushed by us or any other mortal. LOL

I guess I do mine similar, but lazier. I check them in late March to make sure they aren't already growing since they are in my attic that will warm up by that time. In my zone5a I look at 5/20 or so being the day they can go in the ground outside, so if I start them 6-7 weeks early inside that's 4/1 roughly. I put them all in planters on my porch in a 50% potting soil and 50% peat mixture - for me is very loose and easier to pull the plants apart to plant outside. My porch is probably 10-20 degrees warmer than outside for that time of year, so they grow slow.

Gotta go shopping for lights. :o)