Plant up your seeds into pots / seed trays you already have or into containers you find in your recycling bin! There really is no need to go out and buy new pots for seedlings.
Potting compost needs to be PEAT FREE - do check this as you don't want to be encouraging the destruction of peat habitats (which are a vital store of carbon). Homebase Peat Free Multi-Purpose Compost does well in Which? report tests.
Make sure that you sit any cardboard egg boxes / loo rolls on or in something waterproof - or else you will wreck your windowsill (speaking from experience last year, when I had to sand and repaint after the seedlings had vacated the damp and mud-stained sill!)
It is useful to know that six-hole egg boxes fit exactly into the large (1kg and 750g) Lurpack butter tubs. Loo rolls stand nicely in Tetra Pak cartons (on their side with one face removed). To further protect my windowsills, I now like to stand all my seed containers on the upturned lids of A4-paper-sized 'Really Useful' boxes - these act as handy trays and can be easily washed when you want to use them as box lids once more. If you do stand your containers on something waterproof (like plastic box lids) then you can poke a few drainage slits in the base of each junk container, carefully with a kitchen knife.
Label your planted seeds with old lolly sticks or write on brown paper tape with waterproof pen / biro / pencil and stick the tape onto the outside of the tub / Tetra Pak. If you labelled your jars of soaking seeds with brown paper then simply use the same labels - peel them off the jars and stick them onto whatever (waterproof thing) you have planted the seeds in / stood them on.
Water the newly planted seeds a little with lukewarm water. Don't overwater or you will encourage mould to grow and you may waterlog the soil and 'drown' any new shoots (especially if there are no drainage holes which is the risk when using junk potting containers). From then on, only water (and just a small dribble each) when the top of the soil feels dry. The compost in cardboard containers will dry out more rapidly than the compost in plastic and Tetra Pak containers - so check each carefully, don't assume they will all need the same watering schedule.
Once most of the seedlings have germinated (grown a small green shoot and a couple of leaves) then you should start taking them outside to acclimatise to the big, wide world on 'nice' days (i.e. when it's not windy / rainy / cold) for at least two weeks. On pleasant days like this, park your seedlings in a sunny spot in the garden first thing in the morning when you get up and leave them there until just before bedtime. They will enjoy the light from all angles (not just through the window which causes them to bend towards the light) and they will become 'hardened off' to outside conditions. They will then grow more vigorously when you do plant them outside (once all frost risk has passed, so probably in June in most parts of the UK). If you want to know more about the 'hardening off process' and why to go to the bother of taking seedlings on a daily outside field-trip for at least two weeks prior to planting out, then watch #pottopickle episode 10 - part of a brilliant series of gardening videos.
If during these two weeks of being transported inside and outside daily, the seedlings are looking cramped then simply pot them into larger recycling-bin junk, so they have more room for their roots and so that they have fresh compost to feed off (the compost nutrients will only last for three weeks at most).
Oh and once you have finished with your junk gardening 'pots' be sure to rinse the soil off before putting them into the recycling bin (or saving them for next year, which is what I do with the butter tubs).
Happy junk gardening!