If the blanket does not have an outer layer, or you are washing the inner, it is important you know what it is made of. Washing instructions can differ for blankets of different materials. Blankets that weigh more than 12lbs (5.5kg) should be washed in a commercial washer with the capacity for large loads.  Check what your washing machine's weight capacity is too.  If your blanket is too heavy for your washer's recommended load, you can take it to a laundromat or a professional laundering service with large commercial machines. If you are using a professional service, ensure the blanket is washed according to the right temperature for your blanket's fabric. Make sure you do not dry clean your blanket. Select either a cold or warm water cycle depending on the fabric. Choose the lightest wash setting, either a gentle or delicate setting on your washer. Use a gentle detergent that does not include any bleach or whitening agents.  Soft fleece blankets (also called cuddle fleece or 'Minky', of a soft plush feel) should be washed in a cold water machine wash with a gentle detergent. Avoid fabric softener which can gel the short, soft fibers.  Ultra-soft chenille blankets can be washed in a cold or tepid water machine cycle with gentle detergent.  Poly-pellets or bead inner blankets can be washed in a warm water cycle, but avoid hot water. 100% cotton-inner blankets can use either cold or warm water alone in a delicate machine cycle with gentle detergent.  Wash waterproof blankets in a warm or hot water machine wash as these can be harder to clean. Still avoid bleach or vinegar cleaning products.  If you have a flannel blanket, use fabric softener in a cold or medium water cycle. Alternatively, put 1 cup of white vinegar in the rinse water. Doing either of these will soften the flannel and remove pilling (the bumpy threads that break and clump on the surface of the material).

Summary:
Check the fabric your blanket is made of. Check the size and weight of your blanket. Put your blanket in the appropriate sized washer.