Here are a few technical factors to keep in consideration before starting your puzzle.
A word search will tend to be more difficult on a larger grid than a smaller one, due to the larger search space. However, it will also be harder to fit all the letters onto whatever medium you're creating this word search on, so you need to find a balance.
On the other hand, a smaller word search is harder to create, due to the fact that you need to fit so many words into such a small space and they might need to intersect. Be careful that you don't try and make a grid so small that the words become impossible to place.
As a rule, longer words are easier to find than smaller ones, simply because they're so big. However, they'll also be harder to place on a smaller grid, again because there are so many words they have to intersect with. At the same time, you need to be careful that you don't accidentally include a small word in the grid twice by virtue of the random letters coincidentally falling on that pattern.
For the actual puzzle creation process, you'll want to follow this general procedure:
Place big words first.
Big words are, as a rule, harder to place, because they have so many possible intersections and therefore requirements to fulfill. If you place them onto the grid first, there are more places to put them, and it's more likely the small words will fit onto the grid afterwards.
Look for clever combos.
If you notice that three words would work in a triangle or a group of four, put those three or four words together first. Making combos makes for more compact arrangements, which leave more room for other words to appear in the grid.
If it fails, try again.
You may not be able to get all the words you want onto the grid on the first try. If that happens, restart with the big words in a different position. If you've tried a few times and can't seem to get even half of the words onto the grid, try using a bigger grid or reducing your word list.