How do I know when my young plants (from seed) are ready to go outside?

in Plants
Help! I still don't know what I'm doing!!! Haha!
I sowed some zinnia seeds, sunflower seeds, tomato seeds, cosmo seeds and felicia seeds this year and I have NO IDEA how to tell if / when they are ready to go outside.
I did ask a similar question last month, and I think the main reply had to with frost.
But I am confused: should all my young plants go outside the minute there is no more chance of frost? Is that the main rule here?
Or does it also depend on how young / tall / sturdy they are? Is it a matter of common sense / trial and error or are there some steadfast rules I can follow to know when they're ready?
I have a (very) small greenhouse with a couple of shelves but the plastic sheet around it has a tear.
I also read that it's a good idea to put the plants outside for a while, acclimatise them and then bring them back in in the afternoon. How long would you do this for? A week? Two?
Any advice on this appreciated as I think my little plants are doing quite well and I don't want to damage them.
Pictures coming up too
Thanks
I sowed some zinnia seeds, sunflower seeds, tomato seeds, cosmo seeds and felicia seeds this year and I have NO IDEA how to tell if / when they are ready to go outside.
I did ask a similar question last month, and I think the main reply had to with frost.
But I am confused: should all my young plants go outside the minute there is no more chance of frost? Is that the main rule here?
Or does it also depend on how young / tall / sturdy they are? Is it a matter of common sense / trial and error or are there some steadfast rules I can follow to know when they're ready?
I have a (very) small greenhouse with a couple of shelves but the plastic sheet around it has a tear.
I also read that it's a good idea to put the plants outside for a while, acclimatise them and then bring them back in in the afternoon. How long would you do this for? A week? Two?
Any advice on this appreciated as I think my little plants are doing quite well and I don't want to damage them.
Pictures coming up too

Thanks
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Cause he thinks she melts when he touches her
She knows she's the chocolate girl
Cause she's broken up and swallowed
And wrapped in bits of silver