I have always wanted to know how to do this (and I'm sure mine isn't the only way) but through a little practice, I've come across a somewhat simple method to making seamless vector patterns without headache. This is a VERY useful thing to know, so I wanted to share with you my method, and help you bypass some of the frustration I have long endured.
Instead of zooming in and aligning, zooming out, etc, to try and get your pattern to seam (while pulling your hair out and wasting your time), we are going to use some precise tools to do everything for us. Specifically, Illustrator's Transform Palette, Align Palette, and Blend Tool. I've found that making seamless patterns is much easier in Illustrator than Photoshop. We have more control, and can always take our vector pattern into Photoshop and apply it to artwork that way, at any size, to make Photoshop Pattern presets, or even Photoshop Brushes.
First things First:
We are going to be using the palettes/tools mentioned above to make our pattern, but to make things work, we need to make sure the document is set up correctly. If you are going to follow this tutorial word for word (reccommended) you don't need to worry, but if not, your document size must be an exact square. I'll explain why later on in the tutorial.
Make a new Illustrator document and set the size to 400px X 400px.
Make sure your rulers are set to pixels.
First, if you rulers aren't showing, hit Command + R (Mac) or Control + R (PC) to reveal your rulers. (Or you can go to View > Show Rulers). Now, Right/Control + Click on the ruler itself to change the units to Pixels. The reason we want our rulers to show pixels is because we will be using the Transform Palette. Whatever units your Rulers are set to, is what your Transform Palette will display. (From my experience)
Make A Seamless Vector Pattern:
Create a circle on the artboard and size it:
With your circle selected, use the Transform Palette to set the width and height to 20px. Also, in the Transform Palette, make sure the middle “Reference Point” is selected. This is the point that Illustrator will use to align your shape. Set your X and Y to “0”, and you circle should be perfectly sized and aligned in the upper left corner of your artboard:
Duplicate your circle and align it to the opposite corner.
Use the Transform Palette to align the second circle by changing the X value to 400px. (the width of the document) Note: If you have the correct “reference point” for your shapes, you can use the align palette for this, and some future steps, but the Transform palette is better for visualizing the coordinates and sizes of your shapes. Once you have both of your circles aligned, it should look like this:
Set your Blend Options:
We are going to use illustrator's blend tool to create all the dots in between for or pattern. To do this, we need to set our blend options first. Go to Object > Blend > Blend Options.
Blend Options Dialogue:
This part is really up to you but I chose “Specfied Steps” and set the value to 12. Choose your options and Click OK. (“preview” only works if both shapes are selected)
Apply your Blend:
Now that the Blend Options are set, all you have to do is select both dots, like this:
And hit Command + Option + B(Mac) or Control + Option + B(PC) to apply your blend. Or go to Object > Blend > Make.
Expand your Blend
Now we have or dots spaced perfectly along the top edge of our document. Before we go any further, we need to “expand” or shape to the rest of the dots and get an actual path for the rest of the dots. To do that go to Object > Expand.
You will be prompted with the following dialogue box. Just click OK.
Now your top row of dots should all have a path around them:
Duplicate the Dots and Align to the Bottom
Now that you have your top row of dots for your pattern, we need to make the rest of the dots. To do this we are going to make another blend, exept this time, we're gonna blend the entire row vertically. Duplicate your group of dots and use the Transform Palette to make the X value 200px and the Y value -400px. (you can also use the align palette, and align ‘center' and ‘bottom.') Your artboard should now look like this:
Blend the Rows Vertically
This is where the square document size pays off. Because we make our document a perfect square, we don't have to change the blend settings. You can simply hit the keyboard shortcut or go to Object > Blend > Make, and your rows will be perfectly spaced Vertically AND Horizontally. The width and height of the document are the same, so the same amount of space will be given to a ‘row' as would be given to a ‘dot.' This is what your pattern should look like after you Blend the rows:
Expand your Pattern
Just like we did earlier, go to Object > Expand, then click OK in the dialogue to expand your shape and get a path around each dot. Here is the pattern “expanded.”
Create a 400 X 400px Square
Make a square on the artboard, anywhere, anysize.
Now use the Transform Palette to make the Square 400 X 400px, same as the document size:
Align the square to the artboard.
Open your Align Palette and make sure you have “align to artboard” enabled.
Click these 2 buttons in the Align Palette, to align your square perfectly with the artboard. Make sure it's ON TOP of your pattern.
Your pattern should look like this, with the square on top: (my square is filled with white)
Get Rid of the Excess
This is the easiest part of the tutorial. In order to get rid of the shapes that are hanging off the side of your document, you only need to hit one button in the Pathfiner Palette (Window > Pathfinder), with all shapes selected, aligned, and the square on top.
With both the pattern, AND the Square selected, hit the “Crop: button in the Pathfinder Palette.
Illustrator deletes everything that isn't “underneath” the square, and gets rid of the square at the same time! Here's our perfectly seamless vector result:
From here, you can do virtually anything with your pattern. Take it into Photoshop, Use it as a Pattern Swatch in Illustrator, convert it into a Brush. The possibilities are endless. The One thing, if you can only take one from this tutorial, is to utilize the Transform Palette for accurate alignments, and changing “reference Points” for easier aligning.