C Language: Create single string from several different variable types

I'm trying to do what must be a common objective: save experimental data to SD in CSV format to use in Excel. But I can't find all needed code in tutorials or help examples or StackOverflow Examples in Learn C tutorial, from what I see, only save a hard-coded string.

My code list below is long but that is because of careful comments and long variable names.
Problems are at ???, mainly the string length calculation, how to prepare data, how to append each type of datum to string.

/*  StoreDataStringver22.c
Goal:
Concatenate experimental data to string as CSV
Then append to SD file and transfer to Excel

Notes:
  Data types = int, float, char, string
  Goal is the string "dataFullRecord" w/commas
  Data are each fixed length      
Unsolved issues
  round/truncate ints and floats to defined length
  Convert each type to string
  Concatenate
*/

#include "simpletools.h"                      // Include simple tools
int main()                                    // Main function
{
// Vars to hold data for each trial until written
  int datum1_Int;         // 4 digits always (pad as needed)
  float datum2_Float;     // 5 digits always (need pad) +1 for DP?
  char datum3_Char;       // 1 character always
  char datum4_String[3];  // 3 character always
  // might need constant for Comma or Return

// String to hold all data of one trial
  // ??? Size in bytes of total string
    // int            4
    // float         5  +1 for DP?
    // char         1
    // String[3]   3 plus +1 for terminating 0?
    // Commas  3
    // Return added to string or auto by SD append?
  char dataFullRecord[17]; //no Return character
  
// Get data from experiment - dummy values here
[indent]  datum1_Int = 12; // round/pad to 4 digits
  datum2_Float = 9.876543; // round/pad to 5 digits 
  datum3_Char = "A"; //
  datum4_String[4] = "June"; //always 4 char ?+1 for terminal 0
[/indent]

// Build string
    // how to concatenate?
    // how to round/pad int & float to fixed length?
    // how to convert int & float to string?
    // best way to include commas?
    // need Return character?
  // Attempt #6 error: doesn't know function strCopy 
    strCopy(dataFullRecord[](), cstr(round(datum1_int,3))
    strCopy(dataFullRecord[](), ",")
    strCopy(dataFullRecord[](), cstr(round(datum2_float,4))
    strCopy(dataFullRecord[](), ",")
    strCopy(dataFullRecord[](), datum3_char)
    strCopy(dataFullRecord[](), ",")
    strCopy(dataFullRecord[](), datum4_string[])
}