Import the object to Planet Coaster and Steam Workshop

Save the object to Planet Coaster
Open Windows Explorer and go to your User folder, open the Documents folder, then Frontier Development, and finally Planet Coaster. When you are in the Planet Coaster folder check if you have a folder called TMT. If there are none, create it. Click it open.

Here in this folder you place your downloaded file with extension .pcugc

Check your object in game
Start Planet Coaster.

Load a game or start a new one.

In the menu bar at bottom you will see a cube symbol. This is the symbol for Thememaker's toolkit objects.

Look up your item.

Place it in game.

Checklist to verify your object
Is the default look/color of your object reflecting the one that is shown on the icon?

If your object has flexicolor: does it work? Are the primary colors reflecting the sample colors?

Zoom in and out to your object. Is it in same place and will not rotate or jump in sizes? The texture and colors looks same at any distance?

Rotate your object in all directions.

Place lamps around your object. Check for irregular shadows/light.

Is your object initial rotation correct? Up should be up and what's facing you should be facing you.

Check hitcheck box boundaries. Duplicate and stack.

Is your object grid based? Check that it looks good and behaving like you intended.

Check your texture. Are the textures aligning up properly? Seams looking good?

Look at your .pcugc file size. Is it reasonable for its content?

(will write more....)