Using quarter inch grid paper
Almost as soon as I got back from looking at my house that first time, I got out some quarter-inch grid paper and started making a floor plan and considering what to do with the house. I'm on my third pad of grid paper right now and still planning!
With a pencil, it is less exact than a CAD-type program, however for an old house, it works for me. It's quick and easy to erase and change my ideas but I can still keep a record of how my thought process has evolved. Sometimes early plans work best.
With an old house, exact measurements at this point may not be helpful. For most planning, each quarter inch represents 3 inches, so four make 12 inches. Divide a quarter inch square into 3 sections, and each represents 1 inch. Only divide these when needed with just small light marks.
Divide the quarter inch square into 4 sections, and each represents 3/4 inch (the thickness of most boards in the lumber yard) and two of those represents 1 and 1/2 inch (the thickness of a 2×4). For most other measurements, just get close. Measure to the house.
What that means to me is although you may plan and draw exactly 24 inches, when it is time to do measuring, marking and cutting, make the piece fit the house. Go back to the house, and measure from that. You may find it's actually 24 and 1/8 inches instead.
For more detailed work like cabinets, let each quarter inch grid paper square represent one inch. Divide in half for 1/2 inches and then divide again for 1/4 inches. Smaller fractions can be used later in closeup details. Get the overall look and proportions correct. These matter tremendously.
My two most challenging planning tasks have been designing the cabinets for the kitchen and the box beam ceilings for the dining room and living room. These had to fit their spaces, use lumberyard materials, function properly, and finally after doing all of that, they had to look good too.