Both my sons are lefties (as am I). To me, it was fairly obvious early on that they were using their left hand more. I never tried to force it, just let them find their preference.
DS1 is very left handed/left footed but DS2 is right footed when playing footy.
When they were little, they were both thumb suckers. DS1 sucked his right thumb, leaving his left hand free for colouring, etc but DS2 sucked his left thumb!
I remember my friend's Mum offering DS1 a biscuit, when he was about 8 mths old & he reached for it with his left hand - she moved away and tried to get him to take it with his right hand, stupid woman. 
Although I write left handed, I use my right hand for pouring the kettle, (think that's because the kettle was set right handed when I moved in with (right handed) DH, peeling spuds etc. Yet I use my left for using food tongs or stirring cooking.
I use normal (right handed) scissors but in my left hand!!
No wonder I'm mixed up! 😆