The rules
Generally, to construct the correct Fjinnjikulla word you only need to take an Italian word and change every letter with the corresponding form. Example: fare (to do) = f + ae + r + uvi = faeruvi; sciocco (silly) = sij + oio + ukk + ulla = sijoioukkulla.
You must pay attention to two factors: vowel position and consonant clusters. Sometimes, pronounciation is also a relevant factor (but in Italian pronounciation can be inferred from the spelling nearly always).
In Fjinnjikulla, the only allowed consonant clusters, besides doubled consonants, are -nk- and -nkk-. Unallowed clusters are broken by inserting one ore more -u-. Example: schiocco (snap) = s + (u) + k + ji + oio + ukk + ulla = sukjioioukkulla (cf. sciocco above); scrivere (to write) = s + (u) + k + (u) + r + ji + v + ehe + r + uvi = sukurjiveheruvi.
A doubled consonant cannot be word-initial. To avoid this, you need to prefix it with u-. Example: guardia (guard) = (u) + kk + yy + ae + r + (u) + tt + ji + uvo = ukkyyaeruttjiuvo.
As to vowel position, you must take into account Fjinnjikulla forms, not Italian ones. For this purpose, j and y count as vowels. Example: gelo (freeze) = hy + oio + l + ulla = hyoiolulla; cielo (sky) = hi + ji + oio + l + ulla = hijioiolulla.
Accents are dropped, but all punctuation is written, and apostrophs are also written.