The binds below get copied into a file called config.cfg saved in whereever your steam is installed\steamapps\common\Black Mesa\bms\cfg

After you copy the binds, you just have to hit the u button to go 1/4 speed, the i button to go 1/2 speed and so on.

You may need to enable the console or something like that in the options.

The format will match this format below, copy these binds in:

bind "u" "host_timescale 0.25"
bind "i" "host_timescale 0.5"
bind "o" "host_timescale 0.75"
bind "p" "host_timescale 0.90"
bind "[" "host_timescale 1.0"
bind "]" "host_timescale 2.0"