Typically all fabrics are prone to wrinkles or creases when they are delivered and unboxed; this is simply due to the folding and packaging.
Once they are taken out and installed, allow up to 1-2 weeks and the lines will eventually relax and slowly straighten out.
If you'd like to accelerate this process, there are several options depending on the tools at hand:
1. Iron: Set between warm to medium setting and simply run on the surface, but iron it over a piece of fabric - like a sheet or a towel - instead of applying direct heat on the fabrics. While natural fabrics will be able to accept the direct heat, make sure you do not iron directly on polyesters or synthetic leathers.
2. Hand-held Steamer: You can also use a Hair-Dryer and Water Mist Sprayer instead of a hand held steamer; simply run them over the wrinkled areas adequately. If you're using a modern iron with a steam function, use them against the back of the slipcovers.
It would be best to store the slipcovers in the cloth bag provided to allow the slipcovers to breathe. Plastic packaging/vacuum storage will likely cause mildew or molding.