In Japanese a haiku is traditionally 5-7-5 sound syllables. Haiku are and must be brief. Avoid adverbs (words describing the verb or action) and adjectives (words describing the noun or things). Use modifiers only to make your haiku images more exact and precise. Let us know if that gate is a garden gate, a prison gate or a swinging gate. Many adverbs and adjectives imply judgment (beautiful, graceful, ugly) so by avoiding them, and more importantly -- your own opinion, the haiku is left with images of things just as they are.
By being concrete -- using only images of things we can see, smell, taste, touch or feel -- the haiku writer avoids those traps of Western poetry: abstract ideas such as love, hate, sadness, desire, honor, glory, of which we have had enough. Haiku demands you use your bodily senses instead of your intellect. Forget what you have been taught; write of what you experience with your body. Check your haiku. See if you can draw a picture (at least in your mind) as result of reading each line. If you have a line -- "so that it was there" -- you can be sure it is one to drop or rewrite.
Haiku are simple. Often beginners try to put too much into it. A good rule is to have at least two concrete images, no more than three. Some schools of haiku (think of fish) are happy with a couple images which paint a lovely scene wherein your mind can wander and wonder.
Others are more demanding. They ask that the 2 or 3 images to compare, associate or contrast. Here, if you find your way, you can use your ability to see metaphors and simile. You have to accept that by putting the images side by side, leaving out the words "like" and "as" and you will be letting your reader make the leaps of imagination and understand your unspoken point.
Here comes the real challenge of haiku. To express an image or two so well that the reader "sees" them in his/her mind and then! you add another image that demands a leap or twist so the two previous images are seen in a new relationship (maybe even your metaphor, if you are lucky). An additional twist is to have images plus leap which reveal some deep philosophical truth or ideal without having to speak of it. Poetry is written vision. You have to show new ways of seeing things to be a real poet. Basho shows us "real things" doing "unreal" things which feel real:
such stillness
the shrill of a cicada
pierces rock
Part of learning to write haiku is learning to read them. Read translations of Japanese masters, read early haiku written in your own language and read all the contemporary works you can find. Be picky. Decide if you like a haiku or not, if it speaks to you or not and if it does -- why. By analyzing the why you can discover techniques to help you say in haiku what you are experiencing. Go ahead and imitate haiku you like. Just never publish those poems because they are only exercises. Besides, the best-known haiku cannot be imitated.
Because every image is interrelated, be aware of images that reflect a seasonal feeling or spirit. Springtime is for mornings, blossoms and babies, autumn for dying and old folks and evenings. Try to either know through study of kigo lists (lists of season words) which things are associated with which season or by observation. If you use an image out of season, make certain you are doing it for contrast and not ignorance. We non-Japanese are poorly trained in this and it takes study and practice to do that which they do naturally.
Haiku should have a reverence for life and living. To write from the knowledge that even the dead are "alive", that the ugly has something beautiful in it, that even darkness will change to light, is the haiku spirit. Haiku has humor and there is a delight in word-play and puns and the comic of life. Haiku can be written on any subject as long as the writer refrains from being demeaning or sarcastic. If there are times and people who need to speak of things in this manner there is the limerick.
Haiku can be seen as too "cool", too heartless, too objective. Yes, but then you have the tanka form which allows the addition of your subjective feelings and emotions. Accept that different poetry forms grew out of different situations and therefore have a built-in stance or spirit or uprightness. Be aware of what you are feeling and choose the proper genre for it.
Writing haiku is a discipline and if you are interested in haiku you are seeking more discipline in your life. Go for it. Make rules for yourself and follow them exactly, or break them completely, outgrow them and find new ones. We are all students and no one "really" knows how to write a haiku. That, however, does not stop us from trying...
moving into the sun
the pony takes with him
some mountain shadow