A daybed can solve a multitude of problems. It can provide a way for a home office to double as a guest room without always looking like one; it can be extra seating; or it can be a cozy spot to read or relax. A daybed need not be an expensive investment. You can make one from an unused bed you may have on hand.
Evaluate the Bed
Some beds make better daybeds than others. Since the ideal daybed functions as a couch, for seating comfort a twin bed, augmented by pillows or bolsters to provide back support when the user's feet are on the floor, is best. In addition, if you are planning to use a decorative daybed frame, most if not all of them come in twin sizes only. If you want to use a bigger bed, you will need to use your creativity with pillows and the like to fill up the extra space.
Stabilize the Bed
If your bed is on casters, you may want to put cups under the wheels so the bed doesn't roll around when people sit on it or get up from it. This is particularly important if anyone who uses it has mobility problems.
Decorate the Bed
You may choose to use a decorative daybed frame. These are available in a number of styles, from Victorian to Mission to Contemporary. They give a daybed a nice, complete and furniture-like look and also add some support for the pillows and other items you add. When using these, you can go as frilly or as tailored as you like with the spread and pillows, always keeping in mind what style of bedding is appropriate for the frame you choose. You can also buy special daybed slipcovers that conceal three sides of the bed.
If you don't want to invest in these special pieces, you can still make a great daybed inexpensively. Put the bed in a corner so there is support on two sides, and use pillows and bolsters to create comfortable seating. Upholstery shops and craft stores may carry firm wedge-shaped foam bolsters that will support your back better than loose pillows. You can buy extra bedding (or, say, if there are matching bedspread and curtains, buy some curtains for the fabric) and cover the bolsters with some simple hand or machine sewing. Add throw pillows for extra comfort. One advantage of having the daybed in a corner is that you can use any standard bedspread as a covering without an unsightly exposed side.
Form Follows Function
You should consider the bed's surroundings when deciding how to decorate it. If it's in a bedroom being used as a bed most of the time, any bedding you like is appropriate. But if it's in your home office space being used primarily as a couch, you may want to use bedding that is more tailored and subdued – stripes or plaid, for example, rather than ruffled chintz. Any good bedding store will have dozens of options in every possible style to choose from.