Crafting can be a fairly useful skill, especially if you have a lot of time on your character's hands and want to cut costs or get a day job instead of adventuring or traveling. Crafting is kind of a complicated skill. I mean, you read the rules enough and it is easy to follow, but at first glance it is complicated and borderline nonsensical. You can use it to score some marks working for a week at a time in a business related to your Craft skill, Craft (Leatherworking) for instance works for working at a leather shop or a tanner. That is only really useful if you have eight or twelve or whatever hours a day to spend working in a shop for a week, and most adventurers lack that kind of time spent sitting still.
The main use for Craft is building your own crud to cut costs or when there isn't a shop handy to buy stuff at. There is a lot of nonsense about using ironwood or fabricate and other spells to supplement various things involved with the crafting process, but who does that? The first step of the crafting process is to have tools to use. Now, any crafty minded gentleman can improvise a hammer out of a rock and a anvil out of a bigger one. However, improvised tools impose a -2 penalty to the Craft check made while using them. No human being can carry an entire workshop with them, so basically this means that anything more complicated than making arrows or tightening screws done in the wilderness is done with improvised tools. Proper tools impose no penalty or bonus to checks, while masterwork tools grant a +2 bonus to the checks made when using them.
The next step is to convert the cost of the item into bits (an object that costs 15 marks converts into 150 bits, obviously). This conversion of cost is going to determine how much time it will end up taking to craft the item. The next step is to determine the DC of the item, mainly that is DM fiat. There is a chart in the core book, but it is kind of thin as far as examples, though it does cover all the basics.
The next step is to buy raw materials for a third of the item's base cost (so 5 marks for the object I spoke of above). Now, these are raw materials. You aren't buying an unsharpened blade, a tang, a strip of leather, and a hilt and just putting some screws into it to hold everything together and then using a whetstone on it. You are buying ingots and bars of metal, pieces of leather that need to be stitched and cut into the appropriate shapes, wood that needs to be carved and such. If you are making something even more complicated than a sword, you might need wiring, glass, gears that needed to be ground and shaped to properly fit together, and so on. The materials are raw and crude and the cheap cost associated with purchasing them is offset by the time you'll need to get them into the appropriate shape.
The next step is to make a Craft check with all your penalties or bonuses and whatnot. This check represents a week of work. In my mind, that week of work represents six to eight hours of work a day. Dispute it if you like, but rule 0. The result of the check is multiplied by the DC of the check. If the result is greater than the converted bit value of the object, you finish the object successfully. If your result is less than the bit cost of the object, you partially complete it and continue making one check per week until your results add up to the bit value of the object. If you desire, you can make the check on a daily basis, the conversion is simple, you just divide the check result by the number of days in a week
If you don't do well with your checks and fail the DC of the craft check by four or less, you make no progress this week. If you fail by five or more, you waste some of your raw materials and have to pay half the initial raw materials cost again to get more raw materials.
There are a few other options you have, such as making the object masterwork and the option of increasing the DC by ten to hurry the creation process, but ultimately that is the long and short of the craft skill.