THE ANSWER
Leave the hair long enough to be gripped, stop plucking well in advance, arrive with clean skin and no makeup on the area, and skip caffeine that day. If you take retinoids, Accutane or blood thinners, raise them before booking rather than on the day.
Why
Each of those has a practical reason behind it.
Hair length matters because the practitioner needs to see and grip the hair to follow it into its follicle. Too short and it cannot be worked with; too long and it gets in the way. Most practitioners ask for a few days of growth, though the exact preference varies, so ask.
Not plucking matters most of all. A plucked hair leaves an empty follicle, and an empty follicle cannot be treated. Every hair you pull out before an appointment is a hair that cannot be dealt with at it. Shaving and trimming are fine and do not affect the follicle.
Clean skin and no makeup reduce the chance of anything being pressed into a treated follicle, and let the practitioner see what they are working on.
Caffeine is commonly discouraged on the basis that it increases sensitivity. Practitioners give this advice consistently and it costs nothing to follow.
Medications matter because some affect skin fragility or healing. That conversation belongs at consultation, with your practitioner and where relevant your doctor.
Do I shave before an electrolysis appointment or let it grow?
Let it grow for a few days rather than shaving on the day, so there is enough hair to work with. Ask your practitioner for their preferred length, because it varies with method and area.
How long does the hair need to be?
Long enough to be seen and gripped, which is generally a few days of growth. Confirm with your practitioner rather than assuming.
How long before my appointment should I stop tweezing?
Several weeks, so hair is present in the follicles you want treated. This is the single most important preparation step. See why tweezing makes facial hair worse.
Should I stop retinoids before treatment?
Raise it at consultation. Retinoids affect skin, and practitioners generally want to know about them before treating. Do not stop a prescribed medication on the basis of a web page; ask your practitioner and your doctor.
Can I have a session after sun exposure or a tan?
Generally treatment on recently sun-exposed or burnt skin is avoided. If you have had significant sun on the area, say so.
Should I remove makeup before the appointment?
Yes, from the area being treated. Arriving with clean skin is simplest.
What the evidence says
These are standard preparation instructions given consistently across the field. Specific requirements, particularly hair length and medication timeframes, vary between practitioners, so this page presents them as general practice rather than as fixed rules.
What to do next
Stop plucking the area today, even if your appointment is weeks away. That single change makes more difference than everything else on this page combined.
Bring your medication list to your consultation. See electrolysis consultation and who should not get electrolysis.