You take the ferry Smyril from Torshavn to Suderoy. The ferry Smyril is big so you don´t need to reserve a place in advance. The ferry is sailing twice every day and you pay a ticket onboard the ferry after departure. (You only pay for the journey from Suderoy to Torshavn)
The walking distance from the Ferry Terminal in Suderoy (Krambatangi) is 45 min walk. You have the possibility to take the Bus (no 701) or if you prefer I will pick you up at the Terminal.
On one of the picture of Hilmarsstova there is the sailing schedule of ferry M/S Smyril sailing from Torshavn to Suderoy and there is also a picture of a map showing direction from Ferry Terminal to Hilmarsstova.