How do I make a batch file wait / sleep for some seconds?

There are many sleep utilities you can download and drop into your System32 folder, one is provided with the Windows Server 2003 Resource Kit called sleep.exe.

You can also use the ping trick to wait 15 seconds:

ping -n 15 127.0.0.1 > NUL 2>&1

OR

ping 1.1.1.1 -n 1 -w 15000 > nul