I downloaded the free David pattern from the always handy BurdaStyle, printed it off, stuck it together and cut it out. I didn’t have any of the adjustable sliders on hand so I decided to just guess a size by holding the pattern around my neck and stuck the two pattern pieces together to make one long piece. I got some spare material from the fabric cupboard, folded it over and I pinned it diagonally, as you should always cut bow ties on the bias (I think it's so they don't become misshapen over time).
You will now have two pieces of material. Pin the right sides together and now you're ready to sew. Starting near the centre of the thin middle section, sew around the fabric about 5mm in, trying to stay as straight as possible and stop about an inch before you reach your starting point. This leaves a gap for you to turn it in the right way.
Trim the pointed corners at the end by cutting them diagonal, careful not to get too close to the stitching. This stops a build up of fabric at the corners when you turn it the right way out, which is what you should do next. Using a pencil or something similar, place it at the flat end of the bow tie and push it gently towards the gap you left at the centre and pull out the other side, pressing the pencil end into the corners gently to maximise the pointiness.
Once you've turned it in the right way and are happy with your points iron your bow-tie flat. Be especially careful near your opening, folding them so they sit flat and in line with the rest of the tie before you iron them. Also, if you're using a silky fabric make sure you turn your iron down and cover it with a hanky so it doesn't get ruined by the awesome heat.
Okay, now for the tricky/annoying bit. Grab a needle and a thread that is as close to your bow-tie colour as possible. You need to hand sew up the gap so tie a knot and try to hide it inside the tie and use as small a stitch as you can manage to close it up. It's not a huge deal if you intend to wear it with a collared shirt, but a bit of professionalism is always a good plan.