Tabletop Simulator has a bunch of tools that make it easier to create visually appealing mods. The Gizmo tool is likely the most helpful, with a lot of ways to modify and manipulate objects. However, you probably don't need to know how to use the tool to make a working mod.
The Gizmo tool can be found on the left side of your screen. You can also press F8 on your keyboard. There are 5 types of gizmos available - Move, Rotate, Scale, Volume Scale, and Rotation Value. I'll cover all of them.
There are also two settings - Transform Space and Pivot Point. You likely won't need to change these from their defaults unless you're doing really specific things with your objects.
The Gizmo tool can affect the following things:
ANY object you could already interact with (cards, tiles, dice, boards, etc).
Objects that are locked (you can hover over an object and press L to lock it). It's recommended that you lock objects before Gizmo-ing them to prevent them from freaking out if and when you break the laws of physics with them.
Zones that have been placed with the Zone tool (hidden zones, hand zones, scripting zones, etc). Anything that you can do to an object can also be done to a zone.
The Gizmo tool cannot affect tables. I've made a tutorial on how to make custom tables (using the Gizmo tool, ironically), so that might be of help to you if you need a larger table.
Upon selecting an object or zone to start Gizmo-ing, you'll be greeted with this popup. This allows you to manually edit the position, rotation, and scale values of this particular object or zone. Pressing the white button on the right copies these values to your clipboard. You can press the X in the top-right corner to close the pop-up.
The Move Gizmo is selected by default. As you can imagine, it allows you to move objects across all 3 axes. The object or zone you've selected will have these three arrows appear, intersecting at its center. Click and drag on these arrows to move the object or zone. Additionally, you can click and drag on the little boxes to move your object or zone in two axes at once!
Holding down the Ctrl key while moving an object or zone will "snap" it in place, making movement less precise (which might be a good thing for you).
If you hold down the Shift key while the Move Gizmo is selected, a white box will appear. Click and drag this box to ignore the third dimension when moving your object or zone.
The Rotate Gizmo can be used to reposition your object or zone beyond normal 15-degree increments. Click and drag the colored rings to rotate the object or zone.
Holding down the Ctrl key while rotating an object or zone will limit rotations to 5-degree increments.
The Scale Gizmo may remind you of the Transform tool in various graphic design programs. You can also use the + or - keys to scale objects outside of the Gizmo tool. Use the little cubes to scale various axes of the object or zone. You can also use the triangles to scale two axes at once!
Holding down Ctrl while scaling an object or zone will limit scales to 0.10x increments.
If you hold down the Shift key while the Scale Gizmo is selected, a white box will appear. Click and drag this box to scale all three axes at once.
Volume Scale works similarly to the Scale Gizmo, except it only modifies the object or zone in one direction (left or right, for example) instead of both directions. It's functionally the same besides that.
The Rotation Value Gizmo allows you to set and recall various rotations for an object or zone. When you select it, this Rotations menu will pop up. Pressing the + button opens the below Rotation Value menu. Set the X, Y, and Z values as you see fit (or press the red arrow to copy the current rotation of the object or zone), then press Submit.
Right-clicking the object or zone now should show a new option called "Rotation Value", letting you recall that rotation whenever you want!