Congrats on venturing into that first cardigan. These are exciting times.
Increase at the beginning of the row. Probably one stitch in from the edge is the best place for the increase. Do this at the beginning of the 5th row and then 14 rows later (beginning of the 19th row) and then at the beg of rows 33, 47, 61 etc until you have 73sts total. Say you reach 73sts after the inc at row 61, then you’d inc after 16 more rows or on row 77. That’ll bring you to 74sts total.