How I do my guidelines for my writing is something that I get asked all the time.
The answer: I use a special grid sheet that I create and then draw the lines with a coloured pencil.
I can see the grid lines through the Stillman and Birn Alpha paper but if using thicker paper I simple eyeball it by having the grid page hanging out as shown in this photo. So with thicker paper I am more or less drawing the lines freehand but with the grid sheet as a guide for the spacing.
Over the years my grids have changed. Used to have two sizes and then one time I was working in my moleskine book and conforming to a three column grid. That was fun. I create them using a CAD programme and make the lines thick so easier to see through the pages of my sketchbook. Someone on Instagram had a great suggestion of using Excel or a similiar spreadsheet programme
My current grid is 7mm, but I have also used 6mm and 8mm depending on what feels the best for my handwriting. It’s interesting that my handwriting size has changed over the years – I used to use an EF nib and thererfore wrote smaller and neater!
As for handwriting… here is a simple article from last year