Yes, you can use 3mm thick material. Deviation in material thickness of +- 0.5 mm is permissible.

Each drawing indicates the required minimum size of the laser machine's work area. Check that the working area of your machine corresponds to the specified dimensions. If yes, then the drawing is suitable for your laser machine.

Each drawing indicates the thickness of the material. If it says: material thickness 3 mm, this means that you will need to cut several identical parts of a smaller thickness, and then glue them together so that the final thickness corresponds to 3 mm.
However, there are also drawings for any material thickness. You can work with them as you are used to. Such drawings are optimal for cutting on a plotter.

Each drawing is made for a material of a certain thickness. If you change the size of the parts, they will not be able to connect to each other.
However, you can change the size of the parts by using a different thickness of material. If the material is thinner than specified, you can reduce the drawing. If the material is thicker, increase it. These values are not random! There is a certain formula, which I described in detail in the Read Me file, which is in every archive.
There are also drawings for any material thickness; such drawings can be reduced or enlarged.

No, all the details are placed on one sheet. You should be able to do the simplest things: group or ungroup if you need to cut out only some parts, and not all at once.
You can use the free InkScape app to edit drawings.

There are a huge number of applications in the world. Sometimes there is a conflict: your application displays the drawing differently than how I saved it.
1. Check if there is a "keep size" checkbox when you open the file. Perhaps it's something to do with the settings when opening it.
2. Try importing a file of a different format. If you usually use a svg file, this time open the pdf or dxf, they may open correctly.
3. Write to me about your problem, I know how to fix it. But the decision is individual each time.

I buy the tape at my local hardware store. But these light strips are made in China, they are sold on the Internet on every corner. Just look for "1 meter micro led fairy string light 1m/10 lights waterproof".

If there are instructions, you will see a video in the product card. If there is no video, then there are no instructions.

Each drawing is checked + look at reviews from other buyers.
Most likely, you used the wrong material thickness, changed the size of the parts, or the size was incorrect when opening the file. Please read all previous questions and answers again.
However, if something does not work out for you, do not rush to leave negative feedback. Write to me, I really want everything to work out for you.

Files for downloading are available in your personal account. Here are the detailed instructions: https://help.etsy.com/hc/en-us/articles/115013328108?segment=shopping
If the files do not appear in your account, you will need to contact Etsy support. Etsy provides access for downloading independently after verification of payment, this does not depend on me.
I can also send you the files by email, but it will take some time. I will do this as soon as I get to my work computer, where I store all the archives. If you would like me to send them to you, please send me an email to send them to.