Question HealthBar Max size increase question

So, I'm currently learning how to use UI and already learned how to make a basic health bar using sprites or the default image. But I want to increase the bar size with the maximum life from the player using a sprite as a base.

I tried to play around with the Rect Transform and I sort of got it working, but it was a mess to use, and not really good. Just messing with the values broke the thing up. I tried breaking the bar in different parts manipulating just the middle and adjusting the end position accordently. But it also got a mess.

So, is there a better way to make it? Or just a tutorial that I could follow?
Any tip is very welcome.

Thanks.