Thanks to flexible Total Commander settings, three integration options are possible:

1. Preview by pressing space (or any other button). For selecting items insert works instead of space
2. Add QuickLook button to button bar
3. Use QuickLook as external viewer by pressing ALT+F3

Steps:

1. Extracting portable QuickLook-3.6.10.zip from https://github.com/QL-Win/QuickLook/releases/download/3.6.10/QuickLook-3.6.10.zip to installation folder of Total Commander

%COMMANDER_PATH%\plugins\exe\QuickLook\QuickLook.exe

If you don't need portability better use msi version QuickLook https://github.com/QL-Win/QuickLook/releases/download/3.6.10/QuickLook-3.6.10.msi

Then you can everywhere specify absolute paths like

c:\Program Files\QuickLook\QuickLook.exe

instead of relative paths like %COMMANDER_PATH%

2. In Total Commander - Options - Misc.
   1. Redefine hotkey Space
   2. Create and mapping new user command with name em_QuickLook and params:

Command: %COMMANDER_PATH%\plugins\exe\QuickLook\QuickLook.exe
Parameters: %P%N

Internal Total Commander parameters %P and %N pass to QuickLook the path and file name under the cursor on the active panel.

For selecting items Insert works instead of a Space key.

Done!

Optional: to adding QuickLook button in Total Commander button bar Copy this text:

TOTALCMD#BAR#DATA
em_QuickLook

%COMMANDER_PATH%\plugins\exe\QuickLook\QuickLook.exe
Quick Look (Space key)


10000

and in Total Commander button bar context menu - Paste

To add QuickLook as external (alternative) Viewer in Total Commander go to Options - Edit/View and add specify the path to QuickLook and parameter "%1"

%COMMANDER_PATH%\plugins\exe\QuickLook\QuickLook.exe "%1"