A header line for a twin prime search looks like this where the first number is the value of p to start sieving from.
Create a file header.txt containing this line without a newline at the end. This can the be concatenated to the main sieve file as follows assuming that the sieve file without the header is named 0-120T_nohdr.txt and the file with the header is to be called 0-120T.txt.
From a Command Prompt window type
copy header.txt+0-120T_nohdr.txt 0-120T.txt
From a Terminal window type
cat header.txt 0-120T_nohdr.txt > 0-120T.txt