A Leine is a shirt. It is sometimes referred to as "the Saffron shirt"
Sometimes an undershirt, sometimes the only shirt they were wearing.
One point to bear in mind is that drawstrings in the sleeve of a leine is a contemporary development, coming out of American RenFaires in the 1960-70s
Be careful what advice you take.
What is this? This is The Garbindex - a guide to making garb for recreationists (ie how to sew clothes that will make you look genuinely medieval, when 'playing' with such groups as the SCA, doing LARP - or even making something really genuine for drama.)
Use these resources to find resources that will help you create clothing that looks great - and real!