First, you must have your elements isolated in different layers and saved in PSD file. Then open this file in Illustrator, dialog box appears and ask what you want to do, check -Convert layers to objects, then click OK.
Illustrator opens the file and every one of your elements is placed in a separate layer. Select all of your elements and go to Objects menu/pattern/make.
Now you are in Pattern making mode. From pattern options panel (if it didn't open, you can find it under window menu) choose tile type, and bounding box size. There are other options you can play with if you want. Start arranging the pattern, when you are done, click DONE and you can find your new pattern in swatch panel. Delete or hide your elements and in new layer make a rectangle and fill it with your pattern, then click expand(object menu/expand).
Then go to your layers palette and open the new group. In it, you'll find one clipped group which contains another one or couple more clipped groups, this(or every one of them) is your pattern tile, chose one of them and delete others.
Align your artboard to your pattern tile. Now you have 2 options. The first is to save the file and then open it in Photoshop to render the new pattern which is the easiest way to go.(but you have small amount of RAM, there is possibility not to able to or to take too much time) The second is to export the pattern as PDS, PNG or JPG. If you choose this option after exporting open your pattern in Ps and examine the edges in 12800%. I always get a 1 or 2 px transparent line around the edges, so you will too. Which will be visible as a thin white line where the tile ends, when the pattern repeats. There are 2 ways that I know to deal with this. The first one is to go under the Image menu/canvas size and decrease the width and height of the canvas with 1-2 pixels. The second one is to drag guides to the end of the pattern excluding transparent pixels, then using the marquee tool add a selection, which will stick to guides and them click on crop tool which will stick to selection.
That's it, you have your new pattern. I know that sounds complicated but it's actually very easy when you do it once.
The textile designer which Adobe is testing now is very similar to Illustrator pattern maker, but when I tried it with big files it appears to be laggy and freeze, so I wasn't able to make a pattern with it.
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