The spelling of /ð/ is <th> or <the>. The /ʒ/ sound is spelled <ge> and <su>.
The contrast is between two voiced fricatives, fairly close in the mouth. Both sounds may be troublesome for learners, since there are many languages which do not have close equivalents. However, the contrast between them is so rare that it will not be a problem. The /ð/ sound can occur in any position in the syllable, but /ʒ/ only occurs finally or intervocalically, other than in foreign names or loan words such ai>gendarme. /ʒ/ is in any case a very rare phoneme, found in only 334 words in the source dictionary, and /ð/ is also fairly rare, occurring in only 593 words.
The figure for the mean density is 0.4%. Since the source dictionary did not contain the derived forms leathers—leisures or leathered—leisured the semantic loading is 100%, but that figure is hardly relevant with such a short list.
leather leisure bathe beige