A train around 8:00 a.m. puts you in Lisieux before 10:00 a.m. From the station the Carmel on rue du Carmel, where St. Thérèse lived from April 9, 1888 until her death in 1897 and where her relics rest in the chapel shrine, is a short walk into the center; the Basilica stands on its hill a little farther on, and Les Buissonnets, the family home, is on the far side of the center. All three are walkable, though the Basilica climb is steep enough that some visitors take the small tourist train in summer.
The hours worth planning around: Carmel chapel Masses are at 8:00 a.m. Monday to Saturday, 9:00 a.m. on Sundays, and 11:15 a.m. every day; the Basilica's daily Mass is at 4:30 p.m. in the crypt and 11:00 a.m. on Sunday. Les Buissonnets opens at 10:00 a.m. daily except Sunday mornings, with a midday closure, and last entry at 4:00 p.m. in winter, 5:00 p.m. in March and October, and 6:00 p.m. April to September. The Basilica closes at 6:00 p.m. November to March, 7:00 p.m. April to July and September to October, and 8:00 p.m. in August; its crypt holds the reliquary of Saints Louis and Zélie Martin.
An evening train back around 7:00 p.m. gives a full day. Staying overnight is better if you want the cathedral of Saint-Pierre, where Thérèse attended Mass as a child, and the Carmel's evening prayer; the existing page on how many days to spend in Lisieux weighs that choice. Buy tickets on sncf-connect.com; TER fares are fixed-price and need no advance booking.