This page is a basic tutorial relating to F3X, and how to use it to it's fullest. F3X has 14 modes, which are accessible by keystrokes.
Z - Move. Allows you to move bricks around. Global, Local, and Last allow you to change how the bricks are moved (global position, local position). The increment can be changed.
X - Resize. Self explanatory. The increment can be changed. Normal and Both allow you to change how the brick is resized (adding onto both sides if you resize it in Both mode.)
C - Rotate tool, AKA C-Frame. Rotates bricks. You can change the pivot through the settings (Center, Local, Last). Increment of rotation can be changed.
V - Paint tool. Change color of bricks. You can edit the hue manually by clicking on the paint brush below the color collection.
B - Surface tool. It allows you to change any of the six surfaces on a brick, affecting how it sticks. Glue and Weld allow other bricks to stick to it without any fancy work.
N - Material tool. It allows you to change bricks to one of 20 different materials, and it can also change the Transparency and Reflectance of bricks.
M - Anchor tool. It allows you to anchor or unanchor bricks. Unanchored bricks will fall down, and can be moved without needing a tool, anchored bricks will just sit there like a duck.
K - Collision tool. Makes bricks collidable and noncollidable. Noncollidable bricks can be walked through, while Collidable bricks cannot.
J - New part tool. Creates bricks. You can choose from a Normal brick, Truss, Wedge, Corner, Cylinder, Ball, Seat, Vehicle Seat and Spawn. That's pretty much it.
H - Mesh tool. Allows you to add a mesh to a brick. Addable meshes are Block, Cylinder, File, Head, Sphere, Trapezoid and Wedge. Selecting File prompts you to add in a mesh, and it's corresponding texture, from the Roblox catalog.
G - Texture tool. Adds decals and textures to bricks. Selecting Decal makes the image scale relative to the brick, selecting Texture makes the image repeat after a selected distance.
F - Weld tool. Used to group parts together(note: dosen't work like the lost group tool). When those parts are unanchored, they will move together as if they are one large brick. Note: If they are moved before or after being unanchored, the weld will be broken.
U - Light tool. Adds lights to bricks. You can change the color, brightness, range and type of light used, and you can toggle shadows. See the image for comparison between the 3 types of light(Also Spot Light creates light from an angle, Surface Light creates it from a surface.)
P - Decorate tool. Adds Fire, Smoke and Sparkles to bricks. You can change some settings which I am too lazy to talk about, and you can also change the color if you want some pink sparkles or green fire.
There are also 5 other things but they're mostly what they sound like, so yeah.