Solid modeling consists of three abilities that are used to make new shapes. You can now union, negate parts to do so. You can also separate parts back to the way they were if you don't like what you did although sometimes you may negate/union something and then ROBLOX studio won't be able to fix it so that's why you must save constantly!
A 'Union' is two or more parts together to combine into one new shape. When you union something, both of the parts become a new brick called 'union'. To union, select two or more parts with the 'select tool', then go to 'MODEL' and click union. Keep in mind that trying to union lots of bricks at once may not work. You will need to union less parts until you are able to. Unioned parts are a good way to keep brick count low and lag avoided.
A 'Negative Part' is a part that is ready to union with other parts. Before you can negate something, you must select the part(s) you want to negate with and then select the rest of the parts so that the part(s) become negative parts. If you select all parts first in stead of the part you want to become a negative part, all parts will become negative parts. If you try to negate negative parts, they will be removed from the workspace. Once you have negative parts, select those and then the parts you want negated and click 'Negate' on the right of 'Union'
Separate is used to fix unions and get back the negative parts in negated unions. To separate a union or negated union, select the union(s) and click separate which is found on the right of 'Negate'.