You get a pot or tray.
Pop some compost in.
Sow some seeds.
Water gently.
Pop on windowledges or kitchen tabletops.
Things like onions can go straight into a greenhouse, coldframe, under upturned clear plastic containers as soon as they are germinated. Things like brassicas, lettuces, beetroot, broades etc; once they are an inch or so high.
Peppers and tomatoes - just sow a few and keep them indoors - windowledges during the day and tabletops at night. These won't be sown for a few weeks anyway - Mid Jan to Feb ish...[8 weeks away as mentioned earlier]
Peas for pea shoots - will germinate on windowledges. Also sprouting seeds, and microgreens....all of these you can sow year-round.
You can't all live in tiny flats with no windows and no coldframes and no greenhouses, no access to any facilities at all?
I don't live in a flat, but have limited window sill space and single glazing. The temperature in my back downstairs room, even with radiators on, has been recorded at 10 degrees owing to the size of the windows and 75 year old back door.
I very much doubt that seeds will happily germinate and then grow on, happily surviving the temperature excesses on the window, with cold down draughts in what have been -17C temperatures outside (-6 to -8C has been usual of late).
I don't have a greenhouse or cold frame and therefore can't put early starters out there.
I know from experience that peppers and tomatoes started off on windowsills in April get incredibly leggy from low light levels and because there's nowhere sheltered to grow them on once they are a decent size. I've had tomato plants 2' tall with only 4 or 5 pairs of leaves because they have become etiolated through inadequate light levels - even with DD's foil behind them
Starting them off two months earlier than that, without plans for growing them on, is going to lead to heartache.
All that I was trying to point out was that if you have the facilities, it's great to experiment; but many people don't have the facilities, and there are pitfalls to be aware of so that you don't get disappointed by constant failure and not understanding the horticultural reasons behind it.