There are two small solder pads on the speaker to which you attach the decoder wires. The wires must be soldered, which is going to require a very fine tipped soldering iron. Don’t hold the iron to the speaker any longer than absolutely necessary to get a good solder joint because too much heat can damage it.
White, double-sided foam tape is a good way to fasten the speaker enclosure to the frame.
Steam or diesel install? There are different types of enclosures. Usually with steamers, the speaker is in the tender. The tender shell should seal fairly tight.
Many diesels, the shells fit tight. Some use a baffle with the speaker in the diesel shell. I have some diesels with both arrangements.
Below are some links about speakers. Probably more than you want to know.