How install SQL Server 32Bit on Windows 64Bit

If you have SQL Server installation media that contains both 32-bit and 64-bit version of SQL Server, you can choose to install the 32-bit SQL Server (x86) on 64-bit Windows (x64) - WOW64.
On the Options tab, select x86 Processor Type:

To install 32-bit SQL Server (x86) on 64-bit Windows (x64) - WOW64, from the command prompt, specify the parameter:
X86="True"

For SQL Server Express, you need to use an install file that matches your requirement:
SQLEXPR32_x86 to install SQL Server Express 32-bit on 32-bit Windows.
SQLEXPR_x86 to install SQL Server Express 32-bit on both 32-bit and 64-bit Windows.
SQLEXPR_x64 to install SQL Server Express 64-bit on 64-bit Windows.