Put air pockets and air vents on the return end (leaving water end) of
the baseboard. this way the flow of water carries the air with it.
Also make the return end slightly high, so the air naturally goes to
that end. Bleed the baseboard once in a while till you stop getting
air.

Getting the air out with spigots is a tough way to do it, will seldom
get all of the air out and wastes a lot of heated water. Once you
install the vents, you can use them every season.

Stretch