In the USA an instructor does not have to be qualified (or 'certified') to teach. Different snowsport schools have different qualification requirements for foreign instructors and generally the minimum age for employment is 18. Each resort sets its own pay levels according to qualification and, in practice, most resorts will aim to fill full-time positions with qualified staff - part-time staff fill in the shortfalls at peak times at holidays and week-ends. For example, in New Hampshire BASI's Level 2 Instructor qualification is widely accepted and job application is available online at www.loonmtn.com
However in some resorts in Colorado and Montana, at BASI Level 2 Instructor or Level 3 ISIA, until you are established you are likely to be offered posts teaching in the children's programme.
An instructor must apply to a recognised ski school and obtain a job offer. Once the offer is in place, the ski school will apply in the US for a visa petition, you must wait until you receive this petition approval document before you can make an appointment at the embassy for an interview. This will be for an H2 temporary work visa - which is specific to that ski school. Your passport will be retained by the Embassy and returned by special post with the Visa to your UK address. You are not allowed to work for a different employer other than that shown on the visa.
Information can be found on the Embassy website.
Please note: this year there was a cut off date for Visa's and many were let down at the last hurdle. Apply in plenty of time but you may find these positions in short supply due to the tightening US regulations.
For Loon Mountain in New Hampshire, job applications for the following season must be received no later than 1st August. The Human Resource Dept. there also advises on accommodation needs.