How to create and then use a simple tileable* stripe pattern in Photoshop
This process will create a two-color tileable* pattern of vertical stripes and save it as a .pat Photoshop pattern file.
In Photoshop create a new file: File > New
On the right side of the New Document dialog box make these settings:
Under PRESET DETAILS enter the name striped-bgs
This will become the default name of the new file, although it can be changed when you initially save or export the file.
If you intend to set up many pages with the same initial size and other settings you can save those settings as a preset by clicking on the white rectangle icon with the down-pointing arrow to the right of the PRESET DETAILS name field (which ought to have the name striped-bgs in it now).
Set the size to 200px by 200px
Resolution of 72ppi
Do not change the default white background.
Click Create
Select the Rectangle tool from the Photoshop toolbar.
In the Options bar (the horizontal panel underneath the menu bar):
Set the Stroke color to no stroke (in the color click the white box with the red diagonal line).
For the rectangle Fill, select a color.
Set the width to 50px
Set the height to 200px
Click anywhere in the image area.
The Create Rectangle dialog box will appear.
Make these settings:
The size ought to be the same as what you set in the Options bar: 50 wide, 200 high
Set the radii all to zero ( 0 )
Click OK
Open the Properties panel ( Window > Properties )
In the Transform area near the top of the Properties panel the width ( W ) and height ( H ) ought to be 50 wide, 200 high
Set both X and Y to zero ( 0 ). This will position the top left corner of the rectangle to the top left corner of the image area.
Note that X is the horizontal position and Y is the vertical position.
Select the Move ( V ) tool and click on the rectangle.
Hold down the alt (Windows) or option (macOS) key and also the Shift key and drag the rectangle to the right, and continue to hold the keys down...
This will copy the rectangle and also allow you to position that copy (the copied rectangle is now selected by default).
Tooltips will indicate the current position; try to set it to move right 100px and keep the height at zero ( 0 ) (by holding down the Shift key it ought to do that).
If you were not able to set it to those positions, use the Properties panel to do it: X 100, Y zero ( 0 ).
Double click on the bottom layer's name and change it from Background to bg
With that layer selected, in the menu bar select Layer > Layer Style > Color Overlay
Set the Color and Opacity and click OK.
Save your file.
Go the the menu bar and select Edit > Define pattern
In the Pattern Name dialog box ...give it a name that identifies it accurately.
You can now use that pattern as a fill for shapes by selecting it in the pattern section of the fill picker.
You can also save it as a .pat (pattern preset) file:
Select your pattern(s) by using the fill picker.
Then click on the gear icon on the upper right of the fill selector.
Select Export Selected Patterns.
This will make the selected pattern(s) into a .pat file that can be shared with colleagues or loaded into your pattern selector if needed.
* Tileable means that the image can be placed adjacent to itself on any and all sides and remain a cohesive image with no discernible edges.