My method - buy the cheapest, thinnest paper napkins that you can find at the grocery store, school glue (I use the clear blue, but the white works fine too), and I use a wood skewer to place the seeds.
Open the napkin up, dab a tiny bit of glue on your skewer, pick up a seed with it, and place it on the napkin where you want it. For carrots I put 4 x 4 on each quarter sheet (so 64 total on the napkin.) For lettuce I put 4 x 3 on each quarter sheet. Up to you.
The seed will adhere - then when you're done with the sheet - place a second napkin over top and press it down to adhere the two sheets together using the excess glue from each seed placement. Then write the name of the seeds on the top of the sheet. Drape it on something to let it dry (I use the back of chairs usually.)
There you have it! Ready to plant now or when your garden is ready to go. The next day after they've dried real well, I fold and put away until time to plant.
I rake back the dirt a bit from where I want the seeds to go and line up the napkins on the ground, then cover lightly with dirt. All spaced out and no dealing with those tiny seeds! The glue melts when wet, so water well for a few days to keep them wet.
So far I've made these lettuce mats:
Although beet seeds aren't that small, I use the same method for those so that they are spaced properly and it makes them so easy to plant.
So far I've made mats for:
I space these 3 x 3 on each quarter to allow them room to grow.