The package
https://packages.ubuntu.com/bionic-updates/xserver-xorg-video-nouveau-hwe-18.04
is already installed by default.

I have a /usr/lib/xorg/modules/drivers/nouveau_drv.so file.

lsmod |grep nouveau
nouveau              1851392  21
video                  45056  1 nouveau
i2c_algo_bit           16384  1 nouveau
mxm_wmi                16384  1 nouveau
ttm                   110592  1 nouveau
drm_kms_helper        172032  1 nouveau
drm                   458752  11 drm_kms_helper,ttm,nouveau
wmi                    24576  2 mxm_wmi,nouveau

I dont know how to use something special. I just installed the OS and have the
issues i linked. I am using the LTS release for stability.

It crashed already few times.
It crashed few minutes again. Here the last log:

[  4869.797] (EE) glamor0: GL error: GL_OUT_OF_MEMORY in glTexSubImage
[  4869.797] (WW) glamor: Failed to allocate 455x1 FBO due to GL_OUT_OF_MEMORY.
[  4869.797] (WW) glamor: Expect reduced performance.
[  4869.798] (EE) 
[  4869.798] (EE) Backtrace:
[  4869.798] (EE) 0: /usr/lib/xorg/Xorg (OsLookupColor+0x139) [0x5630e630d459]
[  4869.798] (EE) 1: /lib/x86_64-linux-gnu/libpthread.so.0 (funlockfile+0x50)
[0x7fea4ef6a8df]
[  4869.798] (EE) 2: /usr/lib/xorg/modules/libglamoregl.so
(glamor_get_pixmap_texture+0x7d) [0x7fea4b9c155d]
[  4869.799] (EE) 3: /usr/lib/xorg/modules/libglamoregl.so
(glamor_create_gc+0x78f4) [0x7fea4b9d3644]
[  4869.799] (EE) 4: /usr/lib/xorg/modules/libglamoregl.so
(glamor_create_gc+0x7caf) [0x7fea4b9d3d6f]
[  4869.799] (EE) 5: /usr/lib/xorg/modules/libglamoregl.so
(glamor_create_gc+0x8668) [0x7fea4b9d4888]
[  4869.799] (EE) 6: /usr/lib/xorg/modules/libglamoregl.so
(glamor_create_gc+0xa26f) [0x7fea4b9d807f]
[  4869.799] (EE) 7: /usr/lib/xorg/Xorg (DamageRegionAppend+0x6c6)
[0x5630e6290996]
[  4869.800] (EE) 8: /usr/lib/xorg/modules/libglamoregl.so
(glamor_create_gc+0x1110f) [0x7fea4b9e645f]
[  4869.800] (EE) 9: /usr/lib/xorg/Xorg (AddTraps+0x423d) [0x5630e62895ed]
[  4869.800] (EE) 10: /usr/lib/xorg/Xorg (SendErrorToClient+0x368)
[0x5630e61b0498]
[  4869.800] (EE) 11: /usr/lib/xorg/Xorg (InitFonts+0x3c8) [0x5630e61b4478]
[  4869.800] (EE) 12: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xe7)
[0x7fea4eb81b97]
[  4869.800] (EE) 13: /usr/lib/xorg/Xorg (_start+0x2a) [0x5630e619e13a]
[  4869.800] (EE) 
[  4869.800] (EE) Segmentation fault at address 0x0
[  4869.800] (EE) 
Fatal server error:
[  4869.800] (EE) Caught signal 11 (Segmentation fault). Server aborting