Split into 3 army groups.
Fortify your borders, extend the maginot line north, and station a defensive army all along the border, then push through Italy, first with mountain divisions, then send a spearhead into the soft underbelly of the axis where there won't be many German troops, trying to link up with Poland and possibly knocking out Czechoslovakia, with a small but well equipped elite mobile army of tanks and infantry helping hold Warsaw.(If you can send them to help the Spanish, getting them experience would be a huge help when pushing deep into axis territory.)
Before that though, have a small to mid sized army in North Africa ready to immediately make an offensive into Sicily and up the Italian Peninsula before attacking over the alps would help keep the troops on the French border from getting priority reinforcement....
If nothing else, it would be an epic grand battle plan to draw up!
If you can help Poland defend itself with aid for a while, it would be prudent to use the North Africa army to mop up Italian forces in Africa before they're reinforced with Germans and more Italians, maybe sending a few diversion invasions of Marines and paratroopers into mainland Italy to keep them from sending troops to Africa.
This all assumes that you can get Italy into the war earlier than their historically declaration.
Drawing up super complex battle plans engaging multiple armies on different fronts is going to be sooo fun. That's the thing I've been dreaming about since I saw the battle plan editor. I wish they'd use it on www for other than throwing a simple arrow from frontier to frontier...I'm going to try making super complex battleplans, and try to get it to work smoothly with zero interference at some point...maybe a giant US battle plan aiming at Mexico or Canada, drawing main Army arrows, with smaller arrows trying to encircle cities and large groups of forces. Guess decryption and encryption will be a huge factor...they said you can see enemies battle plans if yours is high enough right? I hope neighboring countries actually make battleplans, even if they don't predict war with certain bordering countries.
Here's a poorly drawn battleplan divided into 3 phases, and 3 armies.
http://postimg.org/image/a2h5evrsr/