I struggled with backhand serve and it took me 3 months just to get the basics. 13 years later I am now confident in my short backhand doubles serve to the “T” and quite confident in a wide serve (to the from of the tram lines).
My technique is simple and I encourage everyone I see struggling with serving to try it too. Put your hands infront of you, relaxed so the shuttle is infront of your non racquet leg hip (i’m right handed left shoulder would be at a 40-50 degree angle). Right hand has a “short” backhand grip for more control. Hold the shuttle by the tip of the feather and angle it diagonally down and right. Aim to hit it with the top 4 horizontal strings.
Give a few practice swings to see what I mean about the position. By hitting it here you shouldn’t be hitting your left wrist or fingers with your racquet frame. I have my left foot forwards, standing at the “T”, but just stand whatever is comfortable to you. This could be square on, or the other foot forward. Just make sure to have part of both feet on the ground so it isn’t a foul.
The objective is to get the shuttle to cross as close to the net chord as possible. Anything higher than the net can be attacked by fast opponents. Remain comfortable, and push with your thumb / wrist. Your racquet elbow and shoulder shouldn’t really move.
If you want to flick the opponent, put a bit more pace into the serve just before impact. Do this if they are being very aggressive and standing close to the net on your serve.
In singles, stand a bit further back from the T, because you have to cover more of the court than in a doubles serve.
*You can watch all the videos you like, but you have to practice it and learn what you see*. The most efficient way would be to find a coach and get them to teach you. Could take 1 hr, could take 40 hrs. There’s no point practicing things that are wrong, so start by learning the right thing and it saves you time in the long term.