IRS Form 8936 is filled out and submitted with the tax return. The IRS publishes a new one each year, usually not until the end of January so you can't file until it is released. Tax softwares like TurboTax typically don't get the new form into their software for a couple of weeks after that, so figure sometime in February. The software will still calculate the credit as the dollar amounts won't change when the new form is published, but you just can't file until they have updated with the 2019 form.
As for qualifying you have to have at least $7,500 in tax liability for the year to benefit from the full credit. This is what confuses a lot of people as they think this is talking about whether they owe tax or get a refund when they do their taxes at the end of the year. But it's referring to the tax amount prior to subtracting withholding (slightly oversimplified but that's the gist of it). So for example if someone who did not purchase a Clarity had X dollars income and after accounting for deductions etc. had a tax liability of $9,000, and had $10,000 withheld during the year, they would get a $1,000 refund. However if they purchased a Clarity they can apply the $7,500 credit to their tax liability, reducing the tax liability to $1,500, and after subtracting the $10,000 withholding their refund would be $8,500, which is $7,500 more than the $1,000 that they would have gotten back if they didn't purchase a Clarity.
For an example of someone who would otherwise have owed tax at the end of the year, if the same person who did not purchase a Clarity and had a tax liability of $9,000 only had $8,000 withheld during the year, they would owe $1,000 when they file their taxes. However if they purchased a Clarity they can apply the $7,500 credit and their tax liability is reduced to $1,500. Subtracting the $8,000 withholding they now get a refund of $6,500. The difference between paying $1,000 at the end of the year and getting a refund of $6,500 is a $7,500 difference.
The only problem is when someone doesn't have at least $7,500 in tax liability, then they won't get the full benefit of the credit. For example if someone who did not purchase Clarity had Y dollars income and after accounting for deductions etc. had a tax liability of $5,000 and had $6,000 withheld during the year, they would get a $1,000 refund. If they purchased a Clarity they can only use $5,000 of the credit as you cannot reduce your tax liability to less than 0. They would get all of their $6,000 withholding back, which is nice but in that case they are only getting $5,000 more back than if they had not bought a Clarity.