You can fold the origami figures either freehand in the air or on a base like a table or another base. It is much easier to fold on a table as in the air because you can fold the figures more exactly. This is quite important by folding difficult figures for example the star. Besides this it is also important to tighten the kerb with your fingernail strongly so that the folding does not open again which can cause an impure folded figure at the end.

Freehand folding:

In this way it is quite difficult to fold one corner on top the other corner. You have to fix both corners between your thumb and your forefinger of the one hand.
With the thumb and the forefinger of the other hand you should run down the middle of the sheet of paper to fixing the center of the crease.
After you have these done use both hands for holding the paper at the center of the to emerging fold. First press the thumb and forefinger of the one hand together and run them to the side. When you have put them back to the center do the same with the thumb and forefinger of the other hand and complete the crease.
Don't forget tightening the folding at the end with your fingernails.

Folding on a table:

Folding figures in this way is much easier because you can fix the corners much better between your forefinger of the one hand and the table.
After you have put the sheet of paper onto a flat surface, bring for exampe two edges together and keep the edges together by using the tumb and the forefinger of the one hand. The forefinger of the other hand will be used for running vertically down the middle to fix the center of the emerging folding.
Now you can this forefinger first to the one side of the folding, then to the other side for completing the crease.
Please tighten the foldign afterwards with your fingernail.