Katakana

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

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

アア aa

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

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

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

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

キュウ kyuu (e.g., 急 'urgent')
etc.

To indicate the long ee sound:

1. In most foreign words, length is shown by adding a length marker ー. However, it is also very common to see the symbol イ 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 イ i is added to a syllable ending in e. 
(Note: Although indicated with an イ i, ei is actually pronounced as a lengthened e sound.)

セイ sei
テイ tei
ヘイ hei
ゲイ gei
etc.

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

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

コウ kō
ソウ sō
ゴウ gō
ジョウ jō
ビョウ byō
etc.
Long ō using オ :
トオイ tooi 'far'
トオリ toori 'road'
オオキイ ookii 'big, large'
コオリ koori 'ice'
etc.

Table 5: Representation of Foreign Sounds