Procedures, rules, eligibility, and contribution amounts vary from company to company. Generally, however, you enroll in a plan by completing a subscription form or agreement that authorizes payroll deductions at a percentage or amount that you set, up to certain limits. You then submit it to the appropriate office or department before the applicable enrollment date (e.g. by the 15th of the month before the beginning of a quarter). Your company may call this the "election" period or the "open enrollment" period.
Some companies let you enroll online at a private corporate website or through an employee stock plan provider. When you enroll, you will also want to set up a brokerage account for receiving the shares at the financial institution designated by your company. To avoid backup withholding at the sale of your ESPP shares, when you set up the account be sure you complete and file IRS Form W-9 (for US tax residents) or IRS Form W-8BEN (for nonresident aliens).
Alert: Once you decide to participate in the ESPP and authorize payroll deductions, you may need to affirmatively revoke this election in the future to avoid being automatically enrolled in new offering periods. Also confirm whether your company automatically enrolls new eligible employees in the ESPP (with the right to withdraw before the purchase date), although this practice is much less common with ESPPs than it is in 401(k) enrollment.
If you want to enroll after the initial enrollment period begins, check with your company for when (or whether) it will permit this.
See a related article for details about the key dates and terms you must know before you participate in an ESPP.