Current/natural hair color is an important factor when dyeing hair ash blonde. Unless transitioning from natural blonde, apply a hair stripper prior to dyeing to prevent hair from turning gray or orange. Do a strand test to determine how long to leave the bleach in to reach your desired color.
If you have dark hair, bleach hair prior to dye application. Remember that bleaching is a process and may require multiple stages to achieve your desired tone. Red pigments are especially difficult to eliminate; if your hair has obvious red tones, consult a professional stylist for the first stage of coloring to minimize damage.
Follow instructions carefully, especially in regards to time specifications. Bleaching is a damaging process and can cause damage to hair if done improperly or left in for too long. After bleaching, follow up with regular use of a quality conditioner to restore your hair's moisture capacity.
Take care in the application process, as hair dye stains skin and materials. Always place a towel on your shoulders, and use gloves while applying dye to hair.