If you installed Bitcoin Core into the default directory, type the following at the command prompt:

C:\Program Files\Bitcoin\daemon\bitcoind -daemon
It will display an error message similar to the one below:

Error: To use the bitcoind or the "-server" option to bitcoin-qt,
you must set a rpcpassword in the configuration file:
C:\Users\Example\AppData\Roaming\Bitcoin\bitcoin.conf
It is recommended you use the following random password:
rpcuser=bitcoinrpc
rpcpassword=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
(you do not need to remember this password)
The username and password MUST NOT be the same.
If the file does not exist, create it with owner-readable-only file permissions.

It is also recommended to set alertnotify so you are notified of problems;
for example: alertnotify=echo %s | mail -s "Bitcoin Alert" admin@foo.com
Pay attention to these particular lines:

C:\Users\<YOUR USER NAME>\AppData\Roaming\Bitcoin\bitcoin.conf
[...]
rpcuser=bitcoinrpc
rpcpassword=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
The first line will actually have your username. The later two lines will include a randomly-generated password that you will need to copy and paste. (Do not use XXXXXXX.)

Leave the command prompt window open and start File Explorer. In File Explorer, open the directory indicated in the message (C:\Users\<YOUR USER NAME>\AppData\Roaming\Bitcoin), right-click on the File Explorer window and choose New → Text file. Name the file bitcoin.conf and then right-click on it and choose to open it in Notepad or your preferred text editor.

In the command prompt, copy the rpcuser and rpcpassword lines. Paste them into the text editor and then save and close the file. By default, you shouldn’t need to change the file permissions.

Return to the command prompt and press the Up arrow key to get the previous command (ending in bitcoind -daemon) and run it again. Bitcoin Core daemon should start and print a message that Bitcoin Core is starting.