As a general guideline, make sure you have all your pieces ahead of time and don't force things into place if they don't want to go.
First, open your computer case. Some computers have a button or buttons that you press to open the box. Some have screws on the rear of the case which must be removed. I just need to remove the screws on the back of the case.
The new motherboard is going to be mounted onto the motherboard tray. Some computers have removable trays, but for our example we re going to use this embedded tray.