So interesting you should ask as this is exactly one of my projects right now.
I've put together a few character sheets in my time For DnD and a few other games in Photoshop. These ended up turning out pretty good once I learned the software. What I ended up doing was stitching together a format that was similar but not the same to official sheets for the game with assets by condensing the page, removing elements that I felt weren't needed and moving them to a second or third page, and then finally exporting them as PDFs so I could make them form fillable.
If you're using Photoshop or similar software you're going to want to find a sheet you really like, and then use that as a sort of grid to adjust the layout for your frankenstein monster. Make the layer a bit more transparent and work on top of that. Now you need to be able to add things to this sheet without messing up that particular layer, so you want to open that same document (provided you like that particular design and style of boxes and sheet area) in another panel/artboard but at a much higher resolution (i went 4x) so you can copy, paste, resize and cut those assets without creating a blurry image as you do so.
tl;dr Learn some photoshop and hack and stitch your favorite sheet you can find together with two of the same document by making one a much higher resolution image and the other the frame work.
No I've since moved on from Photoshop. Not because it doesn't suit my needs, but because I wanted to learn another image software. So I'm onto Adobe Illustrator and Affinity Designer. Which are similar but work with Vectors as opposed to bitmaps. This means I don't have to worry about losing clarity for the shapes and such as I work and resize, but I do have to become a bit more artistic and learn to do very very poor graphic design in order to match or imitate the quality of a sheet produced by someone who is not an amatuer.
This is so far a lot of fun, but being that I came from photoshop my learning curves was not as steep, but it still had major hiccups as I learned new terminolgy and the limitations of the scope of both products. If you want to do it the way the guys who are paid to do it do it, then you're gonna wanna learn this eventually.
tl;dr 2 get a vector image editor and learn that, and develop your artistic ability.