Choose 'load disk 0' from the File menu. Click on Games; load imogen.ssd.
Type, at the BBC prompt, "*!BOOT".
The x and z keys go left and right. Press return while you're moving and you'll jump. Use the cursor keys to move along the icons at the top. You can turn into a monkey or a cat (or a bird, if you can catch it, on one level) by pressing the space bar with that icon selected. Monkeys can climb; cats can jump more. Items you pick up you can select with the cursor and press space to use, only as the wizard. Solve the puzzles to collect the magic thing on each level.
There. Imogen is possibly one of my favourite games ever. Do you promise you'll have a go?