For people with a uterus
The process for egg storage is similar to IVF (In Vitro Fertilisation). You will be prescribed hormones to stimulate egg production, then attend a clinic where the mature eggs will be extracted under general anaesthetic or sedation. Because the extraction process is transvaginal it can be a source of dysphoria, and your clinician should be made aware so that they can ensure you are as comfortable as possible. Once the eggs are extracted they are frozen until they are needed. If you want to carry a child yourself and still have a uterus you may be able to have a fertilised egg re-implanted, or they can be implanted in a co-parent or surrogate.
Egg storage is typically for a maximum of 10 years, however, if you are likely to become infertile as a consequence of your treatment then this can be extended up to 55 years. Your clinician will be able to advise whether you meet the criteria for extended storage. It is important to understand that if you are storing eggs and are not likely to become infertile, as a result of your treatment, then they can only remain in storage for up to a maximum of 10 years.
If you freeze your eggs with a private clinic, it can be very expensive. Make sure you have a fully costed treatment plan and have budgeted accordingly.
Embryo storage is a similar process, where the extracted eggs are fertilised before storage. This may be preferable if, for instance, you are in a relationship in which one or both of you are transitioning, and would like to produce an embryo with your partner before any loss of fertility. If, during IVF fertilisation, multiple successful embryos are produced, you will have the option of freezing the remainder in case the implanting/pregnancy is unsuccessful or you want to have more children in future.
You can read more about freezing eggs in the Human Fertilisation and Embryology Authority guidelines.
For people with testes
The process for storing sperm requires you to attend a specialist clinic where you will be asked to produce a sperm sample. This will be frozen until such time as the sperm is required, at which stage it will be defrosted and used for artificial insemination. If you are unable to produce a sperm sample via ejaculation, you may be able to pursue testicular extraction.
If you have been on blockers to suppress puberty, you will not be able to produce sperm for freezing unless you stop your blockers and allow endogenous puberty to progress. As this could cause unwanted irreversible changes in your body, it should be considered very carefully. As mentioned above, in some countries (Australia, Norway, parts of the US) it is possible to have a biopsy taken of the testes for later maturing, but this process is in its infancy. If you have been on cross-sex hormones (in this case, oestrogen) you will need to stop them for around three months before you can freeze sperm, and some overall decrease in the effectiveness of the sperm is anticipated.
In theory, these facilities can store sperm indefinitely, and it should not be negatively affected based on the length of time it is frozen. Prices for private clinic sperm storage vary from clinic to clinic, and it is important to budget for an initial fee followed by an ongoing storage fee, the cost of which is dependent on the amount of time the sperm is stored.
You can read more about freezing sperm in the Human Fertilisation and Embryology Authority guidelines.