Besides knowing the roman orthography for Holo Taiwanese (explained in this handbook:臺灣閩南語羅馬字拼音方案使用手冊), knowing how to read Taiwanese in characters is key for Taiwanese language study in Taiwan. And at long last, the third batch of standardized Holo Taiwanese characters has been announced by the Ministry of Education's National Languages Committee. You can find it here: 臺灣閩南語推薦用字(第3批)
This document contains the newest 300 characters from the 3rd round of standardization, but also remember the 100 characters adopted May 1, 2008 and 300 characters adopted in May 2007.
For the time being, this is the final batch and these three lists of 700 characters total more or less finish the process of standardizing those previously "hard to pin down" Holo Taiwanese characters. Amendments will be made as required in the future.
I got advance word on August 27 of this impending announcement, mostly due to my relentless stalking of the National Languages Committee ( ).
In addition, the MOE's Taiwanese Southern Min Dictionary of Frequently Used Phrases (臺灣閩南語常用詞辭典) which fully integrates the now standardized orthography, is online and functional. The print version will be published in October.
There is also a similar dictionary for Hakka (臺灣客家語常用詞辭典), an initial list of standardized characters (臺灣客家語書寫推薦用字 (第1批)) and a standardization of romanization as well ().
The online version allows you to search in characters or romanization, with tone marks/numbers or without tones, using either the official standard Taiwanese Roman Orthography (台語羅馬字，簡稱台羅拼音) or its predecessor and main inspiration, Church Romanization (教會羅馬字，亦稱白話字).
The dictionary also includes notes on literary or colloquial readings and example sentences. One thing to note: one thing missing from the dictionary are some very common nouns and phrases; the scope of this dictionary is not yet that ambitious. You won't find Tâi-uân (台灣), kok-ka (國家), Tiong-kok (中國) or kok-tiong (國中) in this dictionary, though you will find the characters listed separately or in phrases such as Kok-li̍p Tâi-uân Gē-su̍t Kàu-io̍k-kuán (國立台灣藝術教育館).
The standardized romanization system used in that dictionary, Taiwanese Roman Orthography , is outlined in the orthography handbook mentioned above (臺灣閩南語羅馬字拼音方案使用手冊). The handbook should allow any competent Southern Min speaker to master the basics of the romanization and tone system within a couple of hours.
The dictionary draws on and is also complemented by two earlier Southern Min character databases, both of which have been updated to reflect orthography standardization and are available in print for about NT$300.
閩南語字彙(一)修訂版 & 閩南語字彙(二)修訂版
The MOE also offers a multi-platform input method that gives only romanization output, built on Open Vanilla, downloadable here (these links are to version 1.2, which may be be updated soon to reflect any additions. You can find the download at the dictionary main page, linked to above):
For character output using romanization input, which is what most people will want, there are a number of sources; I recommend FHL Taigi IME (信望愛台語文輸入法).
Unfortunately, there is little public interest in using the new standardized Holo Taiwanese orthography, either in romanization or character form. So don't expect to see written Holo Taiwanese popping up on your TV subtitles, Karaoke lyrics, cereal box, street signs or story books any time soon.
In fact, I expect the vast majority of Taiwanese people, even those younger ones that are educated with the standardized characters and romanization, to remain unable to effectively read or write their mother tongue. But this is certainly a huge step forward.