Site-to-site VPN with multi WAN

Hello,

There are different locations in different cities and one central location. Central location have a few different internet connection ( fiber, wireless and adsl) and one RB750.

Besides, all of other location almost have setup in different cities. Fiber or wireless lines are main lines and others are backup lines.

I need to connect all of other locations to internet over central location’s mikrotik via site-to-site VPN.

How can i do it ?