Welcome is signed by holding your dominant hand up around shoulder or head level, palm facing up and slightly curved, then bringing it closer towards your center. It is like you are welcoming - or inviting - a guest over. This is the same sign for invite.
Teach your toddler the welcome sign when you are receiving guests at home. This will gradually teach your toddler to be a gracious host to friends and family.
The welcome sign should be taught along with the other flash cards that reflect proper decorum and politeness.