If you are an initial applicant for Global Talent (someone who has not previously held leave in this category, or its predecessor Tier 1 Exceptional Talent) there are two applications stages:
Stage 1 - Application for Endorsement: an endorsing body assesses your skills, abilities and achievements, and advises the Home Office whether you can be endorsed. This stage takes up to 8 weeks to be processed. If your application is considered under 'Fast Track' then you may get an endorsement decision in 3 weeks. You must first Apply for Endorsement on the official Global Talent visa website and then also complete a Tech Nation Visa application form for assessment.
In order for your Stage 1 application to be valid you must submit your Tech Nation application form no more than 15 working days after you submitted your Home Office application. Failure to submit your Tech Nation application form will result in your application being rejected as invalid.
The Home Office will inform you if you have been successful in receiving endorsement from Tech Nation. If you are successfully endorsed, you can proceed to apply for the visa (Stage 2).
Stage 2 - Application for Visa: Tech Nation is not involved in Stage 2 - you must apply on the official Global Talent website within 3 months of receiving your endorsement from Tech Nation (failure to do so may result in your endorsement no longer being valid). The Home Office consider immigration aspects, such as general grounds for refusal, and if you are already in the UK whether you are able to switch into Global Talent. This stage usually takes up to 3 weeks if you're outside the UK, and up to 8 weeks if you're inside the UK.