Two ways:
1) While you're standing on the mat, hold the low bar with both hands in normal overhand grip (as oppose to underhand grip), kick one leg up first towards the direction over the bar. Don't swing your head back, but your whole body, with your head in. When your first leg is pretty much like vertical over the bar (you'll get the feeling of the right timing), you kick over your second leg too. While you're kicking over with your legs you pull on the bar with your arms too. You should end up hips on the bar with arms on each side - straight arms, not bent at the end - actually the only time you bend your arms is at the beginning...
2) You can do the pullup with both feet kicking up at the same time, and pulling on the bar with your arms too at the same time - much harder than the first way - you need a lot more abs muscles for this and arm strength. If you're not tall enough to reach the mat when holding onto the low bar, you can also use this method.
....And actually a third way is good for this Level 1/2 skill....monkey see monkey do! :)
