For example this video shows one of the voltage change-over points in The Netherlands. This is at Zevenbergsche Hoek, near Breda, entering the high-speed line to Rotterdam.
The driver gets the instructions on the ETCS DMI, but there are visual instructions outside as well.
At 1m27s there is this blue/white indicator that the pantographs should be lowered in 300m.
1m37s the driver is cutting off the traction, opening the main circuit breaker and lowering the pantographs.
1m51s shows another blue/white indicator where the pantographs should be lowered.
At 1m58s the actual change-over section starts. This first section is by default dead wire, but can be changed to 1500V DC if necessary (when a train would stop in this section).
At 2m38s there is the actual point where the drivers should configure their trains for the new voltage. This is a short section of permanently grounded wire. Right after that, there is another section of dead wire, but this time it can be changed to 25 kV AC when necessary.
3m02s shows the end of the voltage change-over section. The double overhead wire is changed there for a single one, and is live with 25 kV AC.
Indicators at 3m17 and 3m32 show where the driver is allowed to raise the pantographs again, based on train length (8 and 16 coaches respectively). In this video the driver is raising the pantographs just before the 16 coach indicator, as the train consists of 2 locomotives and 9 coaches.
At 3m50s the driver closes the main circuit breaker, and is gaining speed again from around the 4m10s mark.