You will need

1 x 5mm circular needle (NB. 60cm is fine for larger sizes, but 40cm is good for all sizes. If you use a 60cm circular needle the first round after casting on will be tight, but once the first round is completed it is easy) 

1 x 5.5mm circular needle 

4 stitch markers (small tied loops of yarn work fine for this)

1 set of 5 x 5.5mm Double pointed needles

1 yarn needle (for sewing)

3 (3, 4, 4, 5) x 50gm balls Aran weight yarn. The pattern was designed for Malabrigo Worsted Merino yarn)

Sizes are

XSmall (approx 0-4 months) Hips 16" Rise 14" Inside leg 4"

Small (approx 3-12 months) Hips 18" Rise 16" Inside leg 6"

Medium (approx 9-18 months) Hips 20" Rise 18" Inside leg 8"

Large (approx 15-30 months) Hips 22" Rise 20" Inside leg 10"

X Large (approx 2-4 yrs) Hips 24" Rise 22" Inside leg 11"

Method

Using aran weight yarn, and size 5mm circular needles, cast on 72/80/88/96/104 stitches.

Place a stitch marker to mark the start of your round, and join the round. (If you use a cast on which goes through the back of the previous stitch fully, rather than just one side of the stitch, cast on an extra stitch for each size. Then when you join the round, wrap the extra stitch around the last cast on stick, to make a neat join.) 

Video link to joining the round

Continue the round in K2 P2 rib,

Work 2 more rounds in K2 P2 rib. 

Next round Make cord holes (Click here for a video to help you) *K2, Yfd, K2tog* Repeat to end of round.

Then work 3 further rounds in K2 P2 rib. K3 (2, 4, 3, 2) stitches

Turn and then P6 (4, 8, 6, 4)

Turn and then K11 (10, 14, 13, 12)

Turn and then P16 (16, 20, 20, 20)

Turn and then K 21 (22, 26, 27, 28)

Turn and then P26 (28, 32, 34, 36)

Turn and then K31 (34, 38, 41, 44)

Turn and the P36 (40, 44, 48, 52)

That completes the cord holes

Changing to a 5.5mm circular needle, knit until you reach your stitch marker. Start counting as you knit. Count and K36 (40, 44, 48, 52) and place a second stitch marker, preferably a different colour, so you can differentiate the centre back from the centre front. Maintain the placement of both stitch markers as you knit, so you know where you are in each round.

Continue knitting in the round, until your work measures 2" less than half the final rise you want, measuring from the bottom of the cord holes at the centre FRONT of the soaker.

Knit the next round as far as the front stitch marker.

Slip the marker onto the right hand needle, and increase into the next stitch.

Place another stitch marker before continuing to the back centre stitch marker, and then repeat this procedure. *Knit 1 round. Knit the next round as far as the front stitch markers. Slip 1 marker onto the right hand needle, and increase into the next 2 stitches. Slip the second marker onto the right hand needle, before continuing to the back stitch markers, and then repeat this procedure*

Repeat the last 2 rounds until you have 12 stitches between the stitch markers at both the front and the back.

K as far as the first stitch marker at the front of the longies.

Now you will need to change to your 5.5mm DPN needles. Knit the 12 stitches from the gusset onto one of the DPNs

Knit the next 12 (12,16,16,16) stitches onto a second DPN

Knit the next 12 (16, 12, 16, 20) onto a third DPN

Knit the rest of the stitches as far as the start of the gusset at the back onto a 4th DPN.

Cut the ball of yarn off from the work, leaving about a 6" tail end.

Cut a length of yarn about 3' long, and graft the back gusset and front gusset together. Video link to grafting.  Make sure the last stitches of the gusset are tight at both the front and the back, and then weave all the tail ends of the yarn securely back into the gusset to finish them off. You should now have the stitches for one leg on your DPNs, and the other leg still on your circular needle. 

Starting with the DPN stitches, joining in your yarn ball, knit all the stitches on all 3 DPNs. Make sure you pull the stitch where you change from one DPN to another really tight.

After you have knitted all the stitches (1 round) pick up and knit 1 stitch from the edge of the gusset.

Continue knitting the DPN stitches until your work measures 2" less than the final inside leg measurement you require. 

*P 1 round K 1 round.*

Repeat until the rib sections measures 2" and then bind off.

The second leg is done exactly the same as the first, but the first round you need to divide the stitches onto the DPNs, as the circular needle will not be short enough to continue knitting the leg on it. 

TIP - count the rows you knitted before the rib on the first leg, so you can do exactly the same number of rows for the second leg. Count from the extra stitch you picked up from the gusset.

The cuffs

*P one round,  K one round* Repeat the last 2 rounds until the inside leg measurement is the required length. Bind off in purl.

Repeat the leg steps with the second leg, dividing the stitches which remain on the circular needle evenly between 3 DPNs, and remembering to pick up one stitch from the gusset.

Make a draw cord for the waist.