For the Alpha version, we are adopting a system similar to PF2, wherein a critical failure is if you fail the check by 10 or more. A natural 1 is treated as a result one step worse than what it normally would be, so if, for example, a 1 + your bonuses would normally be a critical success, it is a regular success; if it would be a regular success, it's a failure instead; if it would be a failure, it's a critical failure.
Bear in mind that "critical failure" does not necessarily mean some kind of catastrophe, fumble, or similar disaster. For a lot of checks, a failure is more like "limited/partial success," so a critical failure is just absolutely no benefit at all. A critical failure can sometimes have an actively negative effect, but that's certainly not universally true.