Depends. If you're already on the Jubliee line, you can just take the tube to Southwark, no problems, but if you were on the bakerloo or northern line, I'd just get off at Waterloo station and walk, following the signs to the Old Vic theatre then walk in front of the theatre, turn right, go down the road until you see the sign for Valentine Place. I'm on the Northern Line and changing to the Jubilee line at Waterloo station involves a long enough walk that it makes a lot more sense to just go above ground and walk to Silverprint.
And Transport For London's Journey Planner says it's a 10 minute walk from Southwark and a 15minute walk from Waterloo. It does what I recommend above - take Jubilee if you're already on it, eg from Green Park, but if you're on Northern line, don't bother to change to go the one stop east because it'll take you more than the extra 5 minute walk.