every year in january I start collecting milk and juice cartons. I put a big sign on the fridge for my housemates so they remember. the top gets cut of so they rinse and dry well. check out my little collection on the bookshelf:
Initially I start my seedlings in a plastic container that has holes in the bottom (vegetables and mushrooms get sold in these) and use very light soil for good drainage. as a rule of thumb the seeds get planted twice their width deep into the soil. exceptions are plants that require light for germination (i.e Basil seeds) they get placed on top of the subtrate. l always label the containers with plant name and date. it seems very obvious on the day of sowing, but at least I am never able to remember after a couple of weeks time. the soilt then needs to be moist but not wet.
After the seedlings have germinated I let them gow in their container until the first real leaf shows, the first two leaves are the seed leaves and look different. I put a few holes into the milk cartons, so seedlings don´t get waterlogged and fill the carton with light compost. then my free diy plant containers are ready to go. 
in the picture I was transplanting fennel seeds. but some plants don´t handle transplanting very well (i.e. parsley). they can be sown directly into the container with several seeds. i just pull the extra ones out, once they have germinated.
you can see how tightly the containers line up. we are also lucky and have some double windows, were the cartons fit just in between. plants that need a lot of space like tomatoe seedlings make good use of the entire lenght of the cartons. I will be able to plant them out with well developed deep roots from the start. for smaller plants I just cut the cartons in half. 
this is how every single window sill in the house is going to look until the middle of may, when the last seedlings can move outside. I think it is pretty.
commonredstart 