The fastest developments in concentration often come from kasina practice.
Take a circular object of a single plain color. It could be a bowl, a plate, a beverage coaster, a circle cut out of paper, or whatever you like. It just needs to be a simple circle of one color. Prop it up against a background of a somewhat different color and brightness. Sit comfortably at a distance that allows you to see the kasina easily, without significant eyestrain. Stare at it.
That's pretty much it. You can apply more or less effort at various stages of the practice, and see for yourself what results come with differing balances of effort and relaxation. Don't be surprised for very soft stages of jhana to arise early on in the practice. Basically, the first jhana is when you can successfully tune into the kasina and tune out the surroundings. It feels somewhat exhilarating, though afterwards you might feel a bit tired if the effort was high. (The jhana itself does not usually contain the feeling of fatigue -- you might feel it after exiting the jhana, but only if you worked hard.)
In the second jhana, you aren't focusing on the center or any specific part of the kasina anymore. The switch from first jhana to second jhana comes at the point where you're able to relax your effort in a specific way, and instead of losing focus on the object and falling prey to distraction, you become aware of the entire kasina object at once. The eyes often unfocus when that happens, and the object may seem to stick out from the background. Distractions are actually farther away in the second jhana than they were in the first. This is a good way to tell the difference between losing the first jhana to distraction, and moving from the first to the second jhana.
That should be sufficient explanation to get you started. Stare at the object for about 15-30 minutes at a time, according to a timer that you set at the beginning of practice. Don't worry about time anymore once you've started meditating; let the alarm tell you when you're done. Just stare at the object. Try and notice how sometimes you're distracted, sometimes you're unclear, and then sometimes it becomes easier to focus on the center of the kasina. And once you become familiar with that state, see if there's a particular way you can relax your effort that causes your awareness of the kasina to increase and expand to the entire object. If you use the kasina to get really familiar with the first 4 jhanas, and it will pay off big time when you do insight practice.
EDIT: Wow, that was silly. The first jhana is when you can successfully tune out the background and tune into the kasina, not when you can effortlessly tune out the background and into the kasina.
