You can't - the gardening term for this is etiolated - which means they're not getting enough light, so they get thin and leggy. Increase the light supply, or sow the seeds later.

Even sowing the seeds later--which means more hours of light a day--the seedlings may still get leggy if they don't get enough direct sun. Artificial light might help, but may be a project due to the necessary intensity. Granny's old Loetz lamp can't hack it!

You can do something but it is quite involved.

See my blog on light boxes.