I came across a nice article on how to use these characters on a normal US-En keyboard. The steps for this are briefly as follow (they apply to Linux and KDE only)
1) Go to KMenu->Control Center-> Regional And Accessiblity-> Keyboard Layout.
2) Add US-International layout to the set of layouts, after enabling keyboard layouts.(Do not remove the normal english layout)
3) Now in the panel you will notice an icon showing keyboard layout (only if you have multiple of them )
4) When ever composing internal language text, select the US-internaltion layout.
5) For entering short accent keys like
á é ú í ó just press the single quote key before pressing the alphabet
6) for grave accent keys like
â ê ô û î press the exponent key (Shift and key containing number 6 on main board) before pressing the alphabet. Similarly for getting characters like ü ö ë ï ä you can press Shift + " + key(u,o etc.)
7) For typing keys like cedilla ç press Right ALT+Comma
8) For quote ' " you will need to press space key after the normal keys (since they are behaving as modifier key now)
There are many more characters that can be obtained using further combinatios of keys. Some internal of these key mapping are stored in the files in the directory: /etc/X11/xkb/symbols/pc/
For command line you may use this to set the keyboard layout to us_intl and back to us-en
setxkbmap -model pc104 -layout us_intl -variant basic
setxkbmap -model pc104 -layout us