In most cases you do not need to add a new keyboard layout.
Handy keyboard shortcuts accent marks and additional punctuation:
Most symbols and accented letters can be typed by including the option key.
Accented vowels (á,é,í,ó,ú) - option+e, release option and press the corresponding key (either upper or lower case).
ñ - option+n, release then either upper or lowercase N
¡ - option+1
¿ - option+shift+?
Important note regarding letters: if capslock is engaged, this will not put the mark over the letter. The only way to get capitalized vowels is by first pressing option+e, and then you can hold shift while pressing the letter to get the capital version.
Use the "onscreen keyboard" feature to view all possible letters
Click on the Keyboard language switcher in the top menu bar, and then click on "Show Keyboard viewer". This will show you all the available characters and if you press option or option+shift, the on screen keyboard will update with those so you can see what they are.
Other handy symbols available in the US international layout
«» Double arrows (indicating moving between things/processes) - option+\, forward arrows option+shift+\
Cents (¢) - option+4
Euros (€) - option+shift+2
trademark (™) - option+2