Good spelling and grammar checkers are available for Finnish, and letting a program check your text as you type it or after you have typed it may help a lot. Some people even let a word processor or other software auto-correct words. Others may prefer typing undisturbed and check the text separately, perhaps after all content revision and editing has been completed. This is a matter of personal preferences and working style.
There are several checkers available, including the following:
The checkers listed above are illustrated below, using the following sample text, which has two simple misspellings and one grammar error (use of singular instead of plural):
Ilmaus, jonka mukaan julkiset menot on pian 60 prosenttia bruttokansantuotteesta, on tarkkaan ottaen virheellinen. Julkiset menot eivät ole bruttokansantuottteen osa. Tämsällisessä kielenkäytössä olisi puhuttava julkisten menojen suhteesta bruttokansantuotteeseen.
The language pack for Finnish in Microsoft Word has an advanced spelling and grammar checker, with some style checking features. Its settings are described in section Style settings for Finnish in Word of this book.
You may have Finnish language support enabled in Word, or you may need to download and activate it. For instructions on activating the checker, either “as you type” or as a separate run, consult the documentation of Word, as this tends to vary from one version to another.
The screen shot below is from Word 2013, English version.

The screen shot illustrates how Word detects both misspellings, one with three t’s instead of two and one with letters “sm” incorrectly interchanged. It also suggests the correct spellings when you click on a misspelled word, though it often shows a list of alternatives.
It does not detect the number congruence error in the first sentence. In a simpler sentence, where the incorrect formulation is in a main clause, as in “Julkiset menot on...”, Word detects this (when configured to do such checking) and suggests that “menot on” be corrected to “menot ovat”.
Voikko is available from voikko.puimula.org. As mentioned above, it can be used in different application programs. The screen shot below shows its use in the Firefox browser (Finnish version), in composing a message to a discussion forum at WordReference.com. The forum has its own “rich text” box, but within in, the spelling checks work the same way as in simple text boxes.
Firefox does not recognize the language of the text being entered. As a user, you can control its functions by right-clicking on any area where input is possible and selecting the language from the contex menu. In the example, you would select Kielet and then the language. The entry Oikolue lets you select whether spelling checking is enabled.
Voikko detects the same misspellings as Word. It does not detect the congruence error even if it appears in a simple sentence Julkiset menot on...
Oikofix is an online service at Oikofix.com. The default user interface language is Finnish, but English can be selected with a button. This makes American English the default language of the text to be checked, so you need to select Finnish from a dropdown. In the dropdown menu, “Finnish” means standard Finnish, general language. You can also select a version of Finnish with added vocabulary from the natural sciences and medicine and a version with added vocabulary from dialects and old written language.
Similarly to the use of Oikofix for analyzing word forms, the user interface is somewhat special. Instead of first entering text and then selecting a function, you need to
When you click on a word that Oikofix has underlined to indicate a potential problem, it usually shows you one or more suggestions for corrections. Any corrections you make this way will affect only the copy of your text in the text area, so to save the corrected text, you need to copy and paste it. Oikofix also shows the option “Skip error” (Ohita virhe), which causes the skipping of all occurrences of this error during the session.
If a spelling checker lets you add words to a custom dictionary, you probably need to add words in all inflected forms separately. This is not as bad as it sounds, since typically a word occurs in a text in a few inflected forms only, and you can add just forms that you anticipate to be used in your texts.
The frequency of case forms varies greatly, and you might make decisions based on general information on this. For example, if the word denotes an abstract concept, you could include just the nominative, genitive, and partitive forms in singular, at least for a starter.
The Finnish noun oikoluku and the corresponding verb oikolukea are used in varying meanings, with even more variation than the English words “proofreading” and “to proofread”. Whenever an agreement, legal or informal, is made about oikoluku, the meaning of the word should be clarified.
Originally oikoluku means that someone, often the original author, carefully reads and checks typeset text to see that no text was lost or distorted in the typesetting process, that word divisions are acceptable, etc.
Nowadays, oikoluku much more often means checking spelling, grammar, and maybe style. It may well extend to checking the content, perhaps even focusing on it. In computer software, words like oikolukuohjelma (spellcheck program) and oikolukutoiminto (spellcheck function) are used, along with names like kielentarkistus (language checking).
According to language authorities, the word oikoluku should not be used at all. Instead, oikaisuluku or korjausluku should be used, in the original meaning of oikoluku.