Samsung Galaxy S2 has three different boot method.
Normal boot method done by using power button.
You can also hard shutdown the device by holding the power button for 8 seconds, so you don’t need to do battery removal method.
Download mode is necessary for you to flash ROM or kernel.
To perform this boot method, you need to turn off the device.
You need to press and hold volume down, home button, and power button simultaneously.
If you succeed, you will see green download mode on your device.
You can also get into this mode by using USB Jig. You can get the Jig from ebay for few bucks.
Recovery mode, is used to boot recovery such CWM, of course you need to flash the recovery first.
By flashing CF kernel, actually you get rooted devices and also CWM recovery.
To enter this mode, you need to turn off the device first.
You need to press and hold volume up, home button, and power button simultaneously.
If you succeed, after the Samsung Galaxy boot, you will enter the CWM recovery menu.
If you installed custom ROM there should be a reboot into recovery options to get into CWM, so you don’t need to press the button combination