If you mean LDF Key/Values in general, Appendix A of the docs explains that quite well.

[L:4] - number of keys
    [L:1] - key length in bytes
    [L:V] - key, wchar
    [L:1] - data type (see below)
    [L:according to data type] - data

Is the basic structure of a LDF part.

Now there is a list of data types:

0 : String (variable wstring?)
1 : s32
3 : Float (32bit, signed)
5 : u32
7 : Boolean (8bit, 0 or 1)
8 : s64
9 : s64, Used only for (object?) IDs?
13 : bytes?

for each key/value pair that you want to encode, just write
– the length of your key string *2 (bytelength) in one byte
– the key itself with wchar_t’s / as wstring
– the data type as 1 byte
– the data itself

for the data itself, just write them out with the specified length or for variable length data types, prepend the data with a u32/unsigned long length specifier.