Firstly, make a new document. Set the size of it to something small like 1024 pixels on the X and Y axis.
Move your mouse to the top of the screen and click Settings > Configure Krita. A new window will pop up. Type Assistant Tool into the search field. The shortcut for the assistant tool should be empty, so click the empty shortcut space, click Custom, click the box next to it and then type A. It may warn you that another tool is using this key but, for me at least, it was an available key. This isn’t a necessary step but I find myself tapping the A key for the Assistant Tool and the B key for the Brush tool.
Next go to Settings > Dockers then select Grids and Guides. A new small window will pop up. Make sure both Show grid and Snap to grid are ticked. Personally I have my spacing settings at 20 pixels on the X and Y axis.
To test the snapping, tap the newly-assigned A key (or select the Assistant Tool from the left side bar), then go to Tool Options and select Parallel Ruler. Plot the first point as close as you can to one of the corners of the grid squares and plot the second point as close as you can to the next corner. When the ruler has been created re-select the first point and drag it around a bit until it starts to snap to the corner, and repeat for the second. You can make the ruler horizontal, vertical or even diagonal. I tested this method with every assistant tool available and they all snap perfectly.
I hope you guys find this helpful. If you’re experiencing any trouble please leave a comment below.