How to use SDRAM on DE2 board?

All the development boards, including all the DE2 variants, come with a number of example projects. Between them these typically make use of all the peripherals available on the board. One or more will include an SDRAM controller and, more than likely, will be captured as part of a Qsys (or possibly SOPC builder, Qsys' predecessor) system.

So, open one of these projects and explore it. I'm sure you'll learn a lot through this.

You can then consider using one of the example projects as a starting point for your own. Copy the project, open it up in Qsys and, if you're keen, remove all the peripherals you don't need. (If you're keen - arguably there's no need to do this. In your project you can simply ignore those peripherals you don't need.) Note: these examples will typically implement a Nios. If that's what you want, so much the better.

Which DE2 board variant are you using? Most also come with SRAM which, if you're intending to access directly from your own rtl, would be easier. If you're hoping to access the SDRAM directly from your own rtl then you will have to get to grips with the Avalon Slave interface. This interface offers you the on-chip interface (to your own rtl) from Altera's SDRAM interface IP. This presents an easy interface for you to connect your logic to. Look through chapter 2 - "sdram controller core" of the "embedded peripherals ip user guide (https://www.altera.com/literature/ug/ug_embedded_ip.pdf)".

Cheers,

Alex