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You will most often be inserting Parts, which are basic blocks. Try not to use them too much. Instead, make your own models. Also, be aware that some free models have scripts called "infected scripts" - they spread outside of the model and affect everything else inside your game. Making the Toolbox sidebar visible will be of great help even if you don't use Free Models. Do this by selecting every block you want to group, right-clicking, and selecting the "Group" option from the menu. File Publish to Roblox. You occasionally have to push Publish to Roblox multiple times. Go to File ➡ Save as ➡, and then type in the name of your place. After that, hit the 'Enter' key to save it. This way, if you accidentally publish after inserting an infected free model, you can load an earlier version without the infected model.
Click "Model" at the top menu bar. Choose the bricks that you want to insert. Be careful when using free models, available by making the Toolbox sidebar visible in the Window menu. Start adding bricks, models, and whatever you want. At some point, you will want to group blocks together to keep everything looking orderly. After you're finished, you can publish your place to Roblox: Remember to save a backup copy occasionally.