You don’t. We all have to start somewhere and the most flexible yogis in the world weren’t born that way, they have practised with upmost commitment over many, many years to become that way. You should never think you can’t practice yoga because you aren’t flexible enough! It simply isn’t the whole point of yoga. Whilst increased flexibility is a great benefit and side effect of yoga it is not the sole purpose of the practice.
Flexibility is simply one benefit out of an endless list of benefits for the body, mind and spirit. The physical part of yoga (asana practice, i.e. the physical postures you do within a class) is actually just one eighth of what actually makes up the lifestyle of a yogi. Being able to bend into a shape that resembles a pretzel is not a pre-requisite of yoga, having a body that breathes is the only requirement.
Don’t be intimidated, because it simply isn’t a necessity. We all must start somewhere and alignment and posture are far more important than whether you can get your leg over your head. The absolute vast majority of people aren’t like the contortionists we now see on social media, your ability will be much closer to the norm.