After reading Craftsters post on Facebook, I felt it was, again, time to give back!! This tutorial is assuming you have basic sewing knowledge.
Firstly, choose a pair of pyjama pants (or any comfy pants at that matter) with elasticised or draw string waist. These are the ones I chose. They are capri length - but you could do any length - from shorts to longs.

Start by turning the pants inside out. Then place one leg inside the other, like this:

Fold the fabric parallel to the selvedge (this takes care of directional fabric). You can fold it the other way if you don't have directional fabric. Place the pants on the two layers of fabric:

Cut the fabric (through both layers of fabric) following the seam lines as a guide plus adding a little extra as you cut. This allows for the fact that the pants aren't laying completely flat and the fabric you are cutting is. Also, when you get to the seat of the pants on the outer leg, you basically cut a straight line from the hip area - DON'T follow the curve in. The reason is that the elastic is pulling in the sides. The same rule applies for the crotch seam. Start cutting at the curve of the crotch seam but basically go in a straight line after the curve (refer to the photo to visualise this). Also you need to allow for a hem at the bottom and an elastic casing at the top.
You will also see that the elastic casing at the top of my ready made pants varies by about an inch. That's because pants are usually 'higher' at the back than they are at the front. This takes the backside into consideration. I don't bother varying the waistline but you could angle the casing at the front if you desire - it gives a better fit. In the case of my pants, it varies by about and inch from the centre back seam to the centre front seam.

How I calculate the casing:
Measure the width of the elastic then double that measurement. Mine was approximately 1 inch wide. I allowed 2 inches for the casing. An explanation of how I use this calculation is further down when it comes time to sew the casing.

This is one leg cut out. Cut another leg using this as a pattern.

Now, there are many ways to construct pants but this is my favourite as you can't go too wrong with this method. Take one leg and sew the outer leg seam. BEFORE SEWING THE INNER LEG SEAM - press the leg hem allowance. It makes sewing the hem so much easier if it's pressed now. Then sew the inner leg seam. REPEAT for the other leg.

See where I pressed the hem first?:

Now, you've got two legs:

Turn one leg right side out:

Place this leg inside the other one:

Sew the crotch seam:

Press the leg hem again. Also make your casing using your favourite method. This is how I do my casings: Remember I added 2 inches from the top of the pants? I press the top down by 2 inches. I then press the raw edges under enough so that the casing is 1 1/4 inches (my elastic is 1 inch). This allows a little 'give' in the casing. Sew the casing down leaving a gap to insert the elastic. Thread elastic through and measure the elastic on your waist/hips (wherever you like to wear your pants). Sew elastic together then sew the casing closed. To avoid the elastic curling and twisting inside the casing, I sew a vertical line at each side seam to secure the elastic.
Tip: It's better to make a casing for your elastic rather than sewing the elastic onto the fabric. Once you start sewing through elastic (especially lengthwise) it starts to lose its elasticity and also stretches the elastic as you are also pulling on the elastic as you stitch it to the fabric.

Turn your pants right side out and press. Voila, pyjama pants made from your favourite pair!!

As always, please feel free to ask any questions or if you would like something explained further.
