In PSE Editor, go to File menu>new> blank file. Enter the dimensions for H&W, Resolution 240px/in, Background color to suit. This will be your canvas.
Go to File menu>Place, and point to Picture file #1, click on Place. You can drag the picture into position, and resize with the corner handles, if needed.
Go to File>place and point to Picture file #2. Place.
Continue with the other pictures.
Each picture will come in on a separate layer. If you wish to delete a layer for some reason, no problem!
Depending on the complexity of your project, you may not be in a position to finish in one sitting. In that case, save (Save as) in PSD file format, as this will preserve the layer structure. If you require JPEG format subsequently, flatten the layers and resave as JPEG --- no layers.