I used a cardigan pattern for this and just joined the front into the round.
Did 10 rows 1x1 rib then 6rows st st at welt, cuffs and neck.
BEFORE casting on for the neck I measure out enough yarn to do the neck so that I don’t have a join there. Once the neck shaping is done, I stop working the body and do the neck, so that I don’t have the yarn for the neck hanging there, then continue with the body.