In this step, we will use <R,U,r2,u2> to expand the 1x1x3 line into a 2x2x3 block. Usually, the best approach is first to expand the 1x1x3 into a 1x2x3 (by attaching a 1x1x3 at DF-DB or LF-LB), and then to expand the 1x2x3 into a 2x2x3 (by attaching a second 1x1x3 at LF-LB or DF-DB).
As a general rule of thumb, this step should take no more than 10 moves in the worst case (5 moves per line). While pretty straightforward, there are some tricks that can be used to increase efficiency. On the examples page, the following solves contain interesting blocks that you might find helpful:
1 uses an E move to insert the first line, allowing the second line to be completed in 3 moves 7 uses a u2 to solve an otherwise awkward second line in 4 moves 8 inserts the first line with an M2 instead of an r2, allowing the second line to be inserted in 1 move 13 inserts the first line with an r2 instead of an M2, allowing the second line to be inserted in 3 moves 18 solves the green 3x2x1 line instead of the blue 3x2x1 line, allowing the block to be built in 5 moves in exchange for harder F2L recognition and a u2 AUF at the end of the solve 19 uses r' E2 to insert the first line while simultaneously pairing a second line edge with its corresponding center 26 uses a lot of things. I don't think this sort of thing is practical in speedsolves, but it's interesting 41 uses a u in place of a U to form both lines simultaneously 42 uses E moves to form both lines simultaneously 48 solves a free 2x2x2 along with the block, essentially skipping straight to last slot of F2L in 13 moves 54 uses an r and E moves to solve the first line while inserting the center piece into the second line