Hi again Luckygirl - back as promised with a few hints.
First collect some seedtrays - no need to buy, just start to wash and keep some of the plastic containers that come with some foods - eg mushrooms, meat etc. Examples in photo attached (I hope!).
In February/March, watch out for seed displays in shops - some supermarkets (Lidl, Aldi and others) have them as well as the hardware/garden shops. I'd suggest you start with big seeds, like nasturtiums, sunflowers, calendula, marigolds. Other easy ones are "love in a mist" (Nigella), cosmos, cornflowers and snapdragon.
You can read the instructions on the back of seed packet, but even if they say you can plant directly into soil, I always start off in trays as when seedlings come up, you won't know if they're weeds. I've been known to carefully nurture seedlings, only to realise after a while that they're weeds! You can also get flowers earlier when planted indoors to start.
You'll need seed compost - Lidl and Aldi stuff is fine - don't buy till you get your seeds. Then take one of your food trays (a smallish one that will fit inside a bigger, shallow one) and make some holes in the bottom for drainage. Fill it with compost, add some water to make it moist but not sodden, then put seeds on top. Spread them fairly thinly - most seed packets have far too many to plant in a small tray. Sprinkle a thin layer of compost on top, and cover with a transparent plastic tray as a lid. Add a label so you know what's in the tray and put on a bright windowsill.
Depending on the seeds (and temperature), they should start to come up in a few days, or may take a week or 2. When they start to appear, just make sure the compost it still moist. You need to wait till they're an inch or so in height and have some proper leaves - the first ones aren't. Then you can "transplant" into small pots. This means carefully lifting the individual seedling - holding by leaves, not stem. You can use the handle of a teaspoon to loosen the compost around the roots first. Make a hole in the compost in the pot and drop the seedling in - gently firming it down. They should then start to grow into proper plants, but you need to keep them indoors or in your dilapidated summer house till risk of frost has gone (mid/end May) and then plant outside in the ground or containers.
Hope this helps - ended up longer than I intended!