Sheath making is a skill just like any other it will take time to get to a good finished product and most likely after you arrive at a level you are happy with you will think I should have paid the 35 pounds isnce you will spend that much learning it...lol
Anyway after having said that first settle ona style of sheath you want and be sure of your measuring technique nothing is more expensive than wasting leather. If yo are looking for a quick easy source for leather I have heard of guys using weight lifting belts from second hand stores.
One style of sheath is a simple stacked sheath simply trace the knie and add a 1/2 inch around the board and cut out he front and back. Here is an example of a stcked sheath I made you can see how it follows the pattern.
Then ther are pouch sheath a lot of people recomend usinf file folder and laying one side of center adn traqcing the knife like above then adding a 1/2" to that find it works on some knives but not so much on knives with thick handles. If i am working on a new design I use tin foill wrap the knife in an envelope and then cut to shape.
Then transfer the template onto card stock for tracing this way i don't take a chance of wasting leather with a mismeasure. Here is a simple pouch style I made a good beginner style choice.
Don't worry abou tthe tooling that comes later.
If you want you can give you pouch style sheath some shape using the same tin foil method but then sewing and evening out the pattern will get dificult.