How to meditate:

When you are physically settled, you can begin to meditate for a period.
Start with a brief prayer, or make the sign of the Cross, to set your intention to spend time with God.
With your eyes closed, become aware of your thoughts.  Let them go and focus on being attentive to the present moment.  This is our meeting ground with God.
Stay in the present by being aware of your breathing and follow your breathing gently.
Notice any sounds you can hear.  Note what they are - traffic, a bird singing, voices - and return to your awareness of the present.
Note also any physical feelings, such as tension or tingling, and any emotional feelings, but let them go and return to following your breath.
Keep letting go of any thoughts, and if you become caught up in daydreaming, refocus on your breathing, and be aware of that.  See your thoughts as dissolving away into nothing.
It may be helpful to repeat a phrase such as "God", "Peace" or "Maranatha", "Come Lord Jesus", every few seconds to help you to be focused on the present.  You can also count your breaths, up to ten and then start again.  Don't be discouraged if you can't get to ten without being distracted.  Just start again.
At the end of the meditation, say a short prayer or make the sign of the cross.  Take a few moments to refocus on your surroundings.
Meditation is always gentle: don't stress yourself by trying to do too much.  Start with 10 - 15 minutes and find what is right for you.  Two periods of meditation are better than trying to do one really long meditation and getting overtired.
Difficulties in Meditation
If you are unwell, physically or mentally, recently bereaved or very stressed, it may not always be possible to meditate.  Don't force yourself. You can rest in God's presence or simply wait until you feel better.
If you struggle with distractions, take heart - everyone must deal with that, particularly at the beginning.  It is the work of meditation to be aware of our distractions and to return to the present, peacefully.
Don't try to judge the success of your efforts to meditate.  Just meditating is what counts.