Choose a color like pink, maroon, red, cream, or taupe. Lilacs and peaches work well too. Satin and brushed cotton are good materials to work with, as they feel good and are comfortable. You could also buy some cushions in similar colors for a relaxed look.
Soft lighting is essential for a romantic look. Again, pick warm colors, and go for simplicity. Buy as many as you need. Place them on a bedside table at either side of the bed, or just one side if your bed is single. Clear fairy lights would also work. String them up above the bed, or drape them across the headboard.
They don't have to be expensive, they could even be posters, so long as they are in good condition. It's better not to have pictures of people hanging on your walls, as they can feel intrusive. Find canvases or posters of famous romantic landmarks, such as the Eiffel Tower, or choose pictures of flowers. Roses are a good choice.
Floral scents and warm scents, such as cinnamon or vanilla are best. It's best not to choose an overpowering scent, as they can cause some people to have headaches and/or nausea. Be careful and do not leave them unattended.