This is not a route for your first Camino. The Nascente is an expedition, meant for pilgrims who have already walked the well-known routes and are looking for the quietest part of Portugal. Long days, few facilities and almost no other pilgrims. That is exactly the point.
From Tavira you head inland, through the Alentejo. Cork oaks, wheat fields, wide skies and whitewashed hill towns. You walk above the Guadiana past Mértola and straight through Évora. After that the route climbs to the granite uplands of the Beira: cooler and hillier. The stage to Fundão has a steep climb of well over 400 metres. You will often sleep in a small guesthouse or above a café, rarely in an albergue. Where you can, book a day ahead. The waymarks are sometimes good and sometimes poor, so take the GPS route with you. On warm days, top up your water at every fountain.
Walk it from March to May, when the fields are in bloom and the water is still flowing. October is also possible. Give July and August a miss: in the Alentejo it is often far too hot then. The Nascente ends in Trancoso. If you want to continue to Santiago, you carry on along the Camino de Torres. From Tavira that is about two months of walking.