How to start stretching properly?

Hello, y'all

As the the title says, i intend to start stretching soon and i am seeking advice from more experienced men/women in that area.

I started to lift almost 2 years ago and have been aware that lifting tends to make you less flexible.
But it has gotten to the point now where i think it's wise for me to start stretching for real.
I have always been the inflexible type.. but it's worse now ^^

I can't reach the floor with straight legs, can't sit with both legs forward without leaning back, can't reach my fingertips behind my back with one arm top and the other form below and my back is starting to bend when doing highbar-squats...

So.. what are your experiences/ tips that you could give me?
How to start stretching properly?
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