I find it interesting as different devices got a different way to put on download/recovery mode
1. Unplug your device from PC if disconnected.
2. Power it off
3. Pull out the battery, SD Card and SIM from your device.
4. Pressing the Volume down and Home/Ok button, Power it on. You should see your screen like in the pic shown below.
5. Now plug your device to your PC to start download ROM.
Happy flashing!!!