Mindfulness is very interesting in that the concept is so simple, but doing it is so hard at first.
A lot of people just automatically complicate mindfulness and make it out to be a set of actions that you have to perform correctly in order to achieve, and this is likely because almost everything we deal with using conscious thought is complex, so people just presume mindfulness is complex also. It's not, though, which is what tripped me up and trips many people up.
There's no magic, no ceremony you have to perform, when people say "pay attention" there's no obscure meaning to it. You literally just feel, see, touch, taste, smell, hear, and sense stuff. That's it. The difficulty in it lies in the fact that you have to strip away a lot of the stuff you add on top and just sense things without any complication.
In my opinion, moments of mindfulness are more important to increase the frequency of than to increase the length of. Becoming mindful more often helps me more than a super-long session of mindfulness. This I think is because problems with operating mindlessly are easily dealt with by a single moment of mindfulness. You don't need 30 minutes of mindfulness to tell you're in a negative thought pattern; you need a single moment to do that.