While you can use the Character Map accessory (Linux, Windows) to occasionally enter a few characters, you will want to install an input method if you intend to input Japanese characters with any frequency.
A popular method on Linux is SCIM + anthy. SCIM provides an input method engine that supports multiple languages, while anthy provides the Japanese-specific components.
You can use a panel button or a hot key to switch between input modes. While in Japanese input mode, you enter rōmaji and the IM converts it to kana or kanji; this occurs directly in whatever application you’re running (terminal, editor, document, web browser, etc.). Alternately, you can pop up a dialog that lets you click on buttons to enter the characters; this works similarly to Character Map, but has many more options and is specific to the language you’re using.
If SCIM and anthy are not already installed, you can browse in your software package manager for these names specifically or look for something similar to “Japanese Language Support”. The actual package names may vary from distribution to distribution.
If you use Fedora 9 or later, you can enter “yum install scim-lang-japanese” at the command line, which will install everything you need.
Microsoft Windows uses the IME (Input Method Editor), which works similarly to SCIM/anthy above. If you have installed Microsoft Office or upgraded Internet Explorer, you may already have some of the appropriate software installed.
For Windows XP, the following steps should get you started. I’ve not installed or used IME myself, so I can’t vouch for the accuracy of this.
To configure support, follow this path: Start -> Control Panel -> Regional and Language Options.
Select the Languages tab, then check “Install files for East Asian Languages”. You will need your Windows Installation (or Recovery) CD to install the files.
On that same tab (Languages), click the “Details…” button to get to “Text Services and Input Languages”.
Select the Settings tab, the press the “Language Bar…” button to get to “Language Bar Settings”.
Here you can set whether the Language Bar is shown on the desktop and/or the taskbar.
Note: extracted and expanded from Introduction.