How to make them depends an what material you use. Poured concrete, red brick paver, gravel/rock.
I had flat-topped edging installed to make some long walkways in my new house. Then I laid 5/8" minus crushed rock at about 4" deep. 5/8" minus has rocks at 5/8" down through sand size. 1/2" minus and 1/4" minus will also work. This is probably the lowest cost walkway. Don't use pea gravel. It doesn't lock together and will be hard to walk on or move lawn mowers and other objects. At my old house I installed pressure treated 2x4s to form the edges. Landscape blocks can also form the edges.
You will need to level the walkway and have it ready. Then decide what you want to use for the material. You can pour, pave, brick, wood (like a deck), rock, sand. Once you decide the type you want, re post for more advice.