Square hijabs are probably the easiest to wear, depending upon the shape of the face. To make it more comfortable and so it doesnt slip around, most of the women wear a headband, or elastic type lace headbands, or what they call umpta in the Levantine countries like Lebanon, Syria and Jordan. It is kind of like a tube of t-shirt like material that you pull on and then adjust like a hairband over your head.
Fold the square into a triangle but let one side be shorter than the other. The longer side will be what shows outside. Holding the hijab at the folded edge, flip it over the head so that the longer triangle is facing out and lay it on your head. Gently pull on the ends of the triangle to make sure that both sides are even.
Then with some of the material covering your forhead, fold in the sides a little and draw it down under your chin and pin it. Depending upon the type of material, safety pins usually get caught up into the materials, so better to look for special pins or use diaper type pins. Some of these can be purchased through islamic clothing stores online. Below is a picture of how some women tie the ends and flip them over the back of the scarf to weigh it down in the back, however this has to be a wide piece of material and should be soft enough to drape well.
Here is an additional picture which was taken from a previous post, and shows how it is pinned under the chin and then one side will be thrown over a shoulder. This particular hijab is a nice size as it will cover the chest properly, is modest in color and design, and can also be tied and flipped over the back on a windy day. However, on the model, it is not covering the forhead, which will allow the hair to show. Therefore, making the frame around the face smaller and folding in the edges at the temples and pinning under the chin will prevent that from happening. Some women who work and need a really foolproof hassel free look, may use color co-ordinated staight pins at the temple area to attach it to the umpta or headband.
This maroon scarf shows the pink umpta or headband and is wrapped and then pinned using a straight pin on the other side of the head. This allows for nice color combinations, is easy to do, is very feminine and covers well.