When an open compound is the first part of a closed compound, the parts of the inner compound are separated by a space as usual, but a hyphen is inserted at the start of the second part of the outer compound. For example, combining the open compound avaimet käteen (literally “keys to hand”) and the word sopimus (contract) we get avaimet käteen -sopimus (turn-key contract). In pronunciation, each word, including the words of the open compound, is pronounced separately, with main stress on its first syllable, though the first and last word may have a somewhat stronger stress than the middle word.
The hyphen in the written form is an an orthographic convention intended to make the expression easier to read. It is often not applied; substandard spellings like avaimet käteen sopimus are frequently seen.
The convention also applies to expressions like Tuntematon sotilas -romaani, where the open compound is the name of a book or another work and the second part is a general noun indicating the type of the work. The name of the work can be longer, too. These expressions are often avoided by using a different order like romaani Tuntematon sotilas or, more clearly, romaani ”Tuntematon sotilas”, which structurally corresponds to English expressions like “the novel The Unknown Soldier”. By convention, such expressions are treated as open compounds.
If an open compound has an attribute that restricts or extends its meaning, the expression as a whole is treated formally as a closed compound. It is written with a space between the parts, but with a hyphen appended to the first part, e.g. Suur- San Francisco (Greater San Francisco). Such spellings look odd to most people, so they are usually avoided. For example, instead of Itä- Sri Lanka (Eastern Sri Lanka), one may write itäinen Sri Lanka, though this is not quite accepted, or Sri Lankan itäosa (Eastern part of Sri Lanka).
You may ask what to do if both parts of a closed compound are open compounds. Such expressions are rather unnatural, but they are needed in some contexts, especially due to some odd naming conventions for products. The official rule is that a hyphen is used either at the end of the first part or at the start of the second part. Thus, combining the trade name Arla Ingman with the descriptive part rasvaton maito (fat-free milk), we should write either Arla Ingman- rasvaton maito or Arla Ingman -rasvaton maito. Needless to say, such conventions are not widely known and applied; in fact, the manufacturer uses Arla Ingman Rasvaton Maito.
In old Finnish usage, a company name was used in the genitive, e.g. Valion rasvaton maito, where Valio is a manufacturer name. Nowadays, companies wish to use their company name or trade name as immutable, in the basic form, and also wish to use a long product name as a trade mark.
In formal prose, it is probably best to use the exact written form of a product name given by the manufacturer, no matter how much it violates the general rules of the language.