Katakana

1. In most foreign words, length is shown by adding a length marker . 

2. In other cases (e.g., katakana renditions of words usually written in characters or hiragana), the symbol a is added to an a-row syllable (rare).

  aa

1. In most foreign words, length is shown by adding a length marker . 

2. In other cases (e.g., katakana renditions of words usually written in characters or hiragana), the symbol i is added to a syllable ending in i.

1. In most foreign words, length is shown by adding a length marker .

2. In other cases (e.g., katakana renditions of words usually written in characters or hiragana), the symbol u is added to a syllable ending in u.

kyuu (e.g., 'urgent')
etc.

To indicate the long ee sound:

1. In most foreign words, length is shown by adding a length marker . However, it is also very common to see the symbol i  added to a syllable ending in e (as shown in no. 2).

2. In other cases (e.g., katakana renditions of words usually written in characters or hiragana), the symbol i is added to a syllable ending in e. 
(Note: Although indicated with an i, ei is actually pronounced as a lengthened e sound.)

sei
tei
hei
gei
etc.

2. In other cases (e.g., katakana renditions of words usually written in characters or hiragana), the symbol u is added to a syllable ending in o.  There are a few exceptional words where the long oo sound is indicated with an o rather than an u.

(Note: This site romanises long o as oo. This runs counter to the recent trend to use ou. The reason for using oo is: (1) there is a distinction between long oo and the true ou sound that needs to be preserved and (2) there is no distinction in pronunciation between the long oo sound indicated with and that indicated with .)

koo
soo
goo
joo
byoo
etc.
Long oo using :
tooi 'far'
toori 'road'
ookii 'big, large'
koori 'ice'
etc.