Download the source files from the SQLite website: https://www.sqlite.org/download.html Get the sqlite-amalgamation ZIP and the sqlite-src ZIP, you will need both.
Extract the sqlite-src ZIP to a temporary folder. Open the sqlite-amalgamation ZIP and copy the sqlite3.h, sqlite3.c and sqlite3ext.h files to clipboard. Open to where you extracted the sqlite-src ZIP with Windows Explorer then go into the ext/misc folder. Paste the 3 files into that folder.
Next, open a visual studio developer x64 prompt and change to the sqlite-src/ext/misc folder. Enter the following command to compile the extension:
cl /I . /LD spellfix.c /link /OUT:spellfix.dll
Import the spellfix.dll into your SqliteStudio using the Extension Manager.
Make sure you compiled it for x64 bit if you are using a 64bit version of sqlite.
You can check the bitness of the dll using
dumpbin /headers spellfix.dll
FILE HEADER VALUES
8664 machine (x64)