If you have a reference image or a pattern then you may use Inkscape for designing such pattern. it will be good if you have and black and white image. This software converts image file to a vector file. you can download the software here, It's free
I think I may explain it better with an example-
I just downloaded this file to use as a reference for the design.
Now open the inkscape and drag the image file in to it.
Now just adjust the dimensions for your sketch.
Go to Path > Trace Bitmap. In the menu, select the brightness cutoff. you may adjust the threshold here. then just place ok and close the menu.
Now just drag the image outside. you may see there are two of the pics now. one which we just dragged out is vector image.
Delete the other image. adjust the vector image accordingly.
Now save file in dxf format. Don't change any setting in next dialog box. Just click ok and that's it
Open your 3d model in solidworks.
select a plane you want that sketch on and go to Insert > DXF/DWG and insert the saved vector file from its location.
Import it as a 2d sketch and click on finish.
You may see the sketch now. although it may require some scaling and moving.
after adjusting its size you may use any required feature you want to use like extrude boss up to a surface or extrude cut offset a surface etc.
That's it. you may watch this video to learn more about inkscape.