It's a three stage build with crafting, request items and items from the homestead, plus one item each time that's a mission reward.
So, let us guide you through all three build stages of the Fort and tell you what you need.
Stage One
Required for Mission Two
4 Treadmills - Crafted
8 Claw Steeper - Requested
12 Muscle Oil - Adult Standard Pigs
6 Fitness Files - Requested
3 Practice Plow - Crafted
1 Physical Exam - Reward from Mission One
Each Treadmill requires 4 Wood Wheels (Requested) and 4 Leather Tracks (Standard Adult Cows)
Each Plow requires 4 Stone Weights (Requested) and 4 Bent Boards (Grass Debris)
Stage Two
Required for Mission Three
8 Plow Harnesses - Crafted
15 Wool Mats - Standard Adult Sheep
8 Flank Masters - Crafted
8 Muscle Measurers - Requested
15 Clown Barrels - Requested
1 Hauling Trophy - Reward from Mission Two
Each Plow Harness requires 4 Plow Collars (Requested) and 4 Tightropes (Flax)
Each Flank Master requires 3 Horny Handles (Requested), 4 Wooden Rings (Lemon Trees) and 6 Log Weights (Requested)
Stage Three
Required for Mission Four
8 Preppin Poultices - Crafted
8 Bull Pens - Crafted
15 Barbells - Requested
20 Weight Benches - Requested
10 Tough Tuft - Burly Bulls
1 Hoof Cap - Reward for Mission Three
Each Preppin Poultice requires 6 Soothin Salts (Requested) and 4 Egg Whites (Standard Adult Chickens)
Each Bull Pen requires 16 Stone Clips, 12 Peg Hookers (both requested) and 4 Applewood Fences (Green Apple Trees)