1. choose a tube block (hollow cylinder) of the desired texture
2. hold it in place with right ctrl key
3. hit the right ALT key (on german keyboard its called "Alt GR") - the top side of the block will turn red
4. use right shift + arrow keys / pgup/dn to alter the shape of the top side
5. experiment a little bit with it and you will find it out. its not difficult - just needs some practice
additional info: if you are not satisfied with the outcome, you can use shift + backspace key to reset the top side. But that works only as long as the AltGR key is activated.
One more info: you can activate and deactivate this mode also via the radial menu.
And also don´t forget to save the settings, if you find one that suits your need, via the radial menu. By that you create your own blockshapes