This is a UK specific answer (it will become apparent why in a moment) and I am assuming that the founder in question already has a product idea and just needs advice on the mechanics of getting a business started.
Company name. Take some time to think about this. Once you have one or two names, check that they are not registered already. Here is a link to the OK Gov company name searhc engine https://find-and-update.company-information.service.gov.uk/company-name-availability
Once you have your company name registered, you need to select a domain provider (lots of choice of domain providers out there). Check what each provider offers and the deals they can give to new members.
Set up your business bank account. I have used high street banks in the past but most recently I used Tide and it was a very painless and user friendly process.
Register your company with Companies House. Allocate all shares (if you have more than 1) to yourself and make sure you sign up for alerts as you do not want to miss any mandatory regulatory filings.
Create a Government Gateway account for your business through the HMRC portal. You will need to inform HMRC of your business and link your UTR (unique tax ref) to your business. HMRC will assign a tax code and advise you of it in writing.
Register for VAT. If your annual revenue is likely to exceed ยฃ85K then you must register for VAT. If your revenue falls below that threshold, you are not obligated to register. Please watch this space as threshold values can and do change.
Set up your work e-mail for you and any employees you may hire. This is done through your domain provider usually but you can use other service providers.
Build your company branding, letterheads and logos. These days you should embrace AI completely and use one of the many logo building AI tools out there. Most recently, I used Smashing Logo but others are available.
Build your website. This is a long process and you must think carefully about the user experience. Work out what you want to say, how you want to say it and the artworks you want to use from your branding acitivity previously. If you are using a different website builder from your domain provider, you will need to enter your domain information into your website builder. This is usually supported with step by step instructions from the website building tool. Most recently I used Wix to build a website. Feel free to take a look at the results at https://www.cartco.org.uk/
Create a LinkedIn page for your new company. Upload your logos, start writing some blogs or thought leadership articles on your website and link them to your new company page on LinkedIn. Invite your connections to follow your new page.
Business Insurance - you will need to get D&O insurance to cover off any liabilities you carry for any wrongful acts you or your business carry out. Depending on your business you may need other types of insurance as well (public liability insurance, medical malpractice insurance etc).
Closing comment:
I used Tide recently to set up my business bank account. As part of the set up service, they also register your company with Companies House and HMRC on your behalf. This is a massive time saving for you and you can relax in the knowledge that your registrations have been completed correctly. All you do is provide the neccesary information and Tide does the rest.
This does not apply to VAT registration. That you must do yourself.