Monstercraft comes from the sourcebook Heliana's Guide to Monster Hunting and here's how it works...
First you Kill... then you Harvest... then you Craft.
After a creature is slain, its components must be harvested quickly before their magic fades. There are three steps involved in this process.
After you have slain the creature, use your copy of Heliana's Guide to Monster Hunting to identify the type of creature you've slain.
Using the appropriate Harvest Table, make a list of the components you'd like to try and harvest.
Based on the harvest list, make your Assessment check and Carving check to see which components you're able to retrieve.
NOTE: Just because a creature belongs to a certain type (i.e. Aberration) doesn't mean it will have every component from the Aberration list. For example, "tentacle" is listed on the Aberration list, but if the creature doesn't have tentacles then that component obviously won't be available from this creature.
Once you've gathered all of your components, it's time to make something amazing... or monstrous....
You have two paths to choose from...
If you are magically inclined, you can extract the magic from the creature parts and imbue them into your item. This process requires you to already have the base item (i.e. - longsword) and uses your spellcasting ability rather than craftsman tools.
For our example, let's make a Dancing Sword (longsword).
This item is considered Rare and requires attunement, which means we'll need a Robust essence, it's Enchanting check will be a DC 18, and it will take us 80-hours to complete.
I already have a longsword that I'm going to enchant, but if I didn't, I could manufacture one myself or I could simply buy one.
According to this item's recipe, we'll need the Lifespark from a Construct creature. And if that creature was specifically a Flying Sword, we'll have advantage on our checks!
Once we have all of our components, we can begin! Usually this would take us about 10-days to enchant, but I'm going to hire a local enchanter to help me cut this time in half. Their hourly rate is 20 gp so I'll give them 800 gp so we can finish in half the time.
And 5-days later, we're finished! Time to make our final checks to determine the results. Since the creature part came from a Construct, if we're proficient in Investigation, we can add our proficiency bonus to our check!
Everyone who worked on the item will make a check (that includes any hired help) and we'll average the checks together.
My Intelligence check was a 15 but I'm proficient in Investigation so I get to add my +3 proficiency bonus, putting me up to a 18 total.
My hired help rolled a 13 and they get a +6 bonus to their roll, putting them at 19.
Our average is 18 (rounded down) so we have succeeded!
For those who are not magically inclined (or if you want to make the item from scratch before Enchanting), you can Forge your item! This process allows the creature parts to be molded into the item upon creation, meaning no magical ability is required.
For our example, let's make a Dancing Sword (longsword).
Since we're making this item from scratch, we'll need the raw material to craft the base item (longsword). This will require ingots worth about 1/3 of the overall value of the longsword (15 GP).
This item is considered Rare and requires attunement, which means we'll need a Robust essence, it's forged enchanting check will be a DC 18, and it will take us 80-hours to complete.
According to this item's recipe, we'll need the Lifespark from a Construct creature. And if that creature was specifically a Flying Sword, we'll have advantage on our checks!
Once we have all of our components, we can begin! Usually this would take us about 10-days to forge, but I want to save some time. I also want this to turn out really well so I'm going to hire a master level forger to help me. His hourly rate is 200 gp, but he's 3x as fast and has more skill! I'll give him 2,667 GP to do half the work and I'll do the other 20-days.
When we're finished, we'll both make two checks to determine the result. First we'll make a manufacturing check and then we'll make a forged enchanting check. Since the creature part used was from a Construct, we can add our proficiency bonus to our forged enchanting check if we're proficient in Investigation.
My Strength check was a 15 and I'm not proficient with Smith's Tools so I cannot add my proficiency bonus.
The hired master forger rolled a 13 but he gets a +11 to his check giving him a 24!
Our manufacturing check average is 19 (rounded down). That's a success on the base item creation.
My Strength check was a 14 and I'm proficient in Investigation so I get to add my +3 proficiency bonus giving me a 17 total!
The hired master forger rolled a 10 but he gets a +11 to his check giving him a 22!
Our forged enchanting check average is 19 (rounded down). That's another success on the forged enchantment!
Note: Anyone can Forge, even those who would prefer to Enchant. You'll just do the first part of the Forge normal and then do your Enchanting afterwards.