For this tutorial you may want to get a Plaid image as reference.
1 - In Illustrator, create a “New Document”.
2 - Select the “Rectangle Tool” (M) to add some squares and fill them with the colors present in your “Plaid Pattern”. You can “Place” the reference image into your “Document” and use the “Eyedropper Tool” (I) to sample the colors. Make sure the “Smart Guides” are ON (Command/Control + U).
3 - Place a couple of “Guides” to mark the Top-Left corner of your “Plaid Pattern”. To do this, just click on the “Rulers” (Command/Control + R) and drag to the spot you want the “Guide” to be placed. Back to the “Rectangle Tool” (M) to create a with as 4 cm “Width” and a 5.5 cm “Height”. Add other two “Guides” to mark the Bottom-Right corner.
4 - Use the “Eyedropper Tool” (I) to fill the rectangle with the base color of your pattern.
5 - Select the “Line Segment Tool” (\) and draw a line from Top to bottom while holding the “Shift” key to make sure its straight. Give it one of the sampled colors and adjust the “Stroke” size to make it similar to the pattern you are using (ours was 10 pt). Place other lines and adjust the size, color and position to match the reference pattern.
6 - You may need to “Swap Fill and Stroke” (Shift + X) in the Toolbar after using the eyedropper to assign a color. Select the portions of the pattern that you want to repeat and drag them to the spot you want to place them while holding the “Shift” and “Alt/Option” keys to create a duplicate and move it in a straight line.
7 - Once you are done, select all the lines of your pattern and go to Object > Expand, make sure that the “Fill” and “Stroke” boxes are checked and click “OK”.
8 - Select all your Pattern and drag it to the side while holding the “Shift” and “Alt/Option” keys to create a duplicate and move it in a straight line. Deselect the base rectangle (hold “Shift” and click on it), rotate the rest of the pattern to make the lines horizontal and reduce their length to fit the base rectangle. Duplicate and/or add more lines to create the Horizontal design of your pattern.
9 - You can get rid of the “Guides” now. Go to View > Guides > Clear Guides. Now, select the first part of the “Plaid Pattern” and “Copy” it (Command/Control + C).
10 - Open “photoshop” and choose “Clipboard”, make sure that the “Width” and “Height” are the same as the ones of your pattern base. Use a 300 dpi “Resolution” and set the “Background” to “Transparent”.
11 - “Paste” (Command/Control + V) the “Plaid Pattern” you copied from Illustrator, choose “Pixels” as “Paste” Method and click “OK”. Check if the image fits the “Canvas” properly then click the Check-mark on the Top Menu or hit “Enter”. Name the layer “Vertical”. “Copy” and “Paste” the other “Plaid Pattern” and name the layer “Horizontal”.
12 - Create a “New” Photoshop Document (Command/Control + N), make it 4 Pixels by 4 Pixels @300 dpi with a “Transparent” background. Zoom-In as much as you can, select the “Brush Tool” (B) with a 1 Pixel 100% “Hardness” brush and paint some pixels with Black in order to create a “line” going from the Top-Left corner to the Bottom-Right one. Since you are working with a very small canvas your line will be jagged and made out of squares (the first 2 in the Top-Left, then leave one on top and bottom blank and paint the middle ones, etc.). Paint the Top-Right square too.
13 - Once you are done, go to Edit > Define Pattern, name it “PlaidPattern” and click “OK”. You can save the small pattern file or delete it if you want.
14 - Click the small Black and White icon at the bottom of the “Layers Panel” and create a “New Pattern Fill Layer”, choose your newly created “PlaidPattern” and click “OK”. Right-click on top of the layer’s thumbnail and choose “Rasterize Layer”.
15 - Use the “Magic Wand Tool” (W) to select the Black lines, turn off the layer’s visibility by clicking on the small eye icon placed on the left side of the thumbnail and select the “Horizontal” layer then, hit “Delete” to get rid of the selected portions and create a “Plaid Pattern”.
16 - Go to File > Save as… to save your file as a Photoshop file and name it “PlaidPs.psd”. You can save it in other formats, but this will let you keep the layers for future usage. Click “OK” when asked to “Maximize Compatibility”.
17 - Before closing your file, go to Edit > Define Pattern, name it “Plaid Pattern Ps” and click “OK”. Now, you can use this pattern in Photoshop it if you want.
18 - Go back to Illustrator and head to File > Place (Shift + Command/Control + P). Navigate to where your “PlaidPs.psd” file was saved and select it making sure that the “Link” and “Show import Options” boxes are checked. Click the “Embed” button on the Top Menu and choose “Flatten Layers to a Single Image”.
19 - Drag the placed Pattern to the “Swatches Panel” and double-click on top of the small swatch icon to open the “Pattern Options”. Rename the swatch “PlaidPs” and click “Done” in the upper Menu. You can use the pattern in Illustrator.