Just wondering when to do these, as i haven't beaten the main questline before. Just don't want to miss something, or miss out on doing them. Not sure how they fit in story wise.

Either of them can technically be done whenever. Dawnguard "officially" activates once you hit level 10, but you can go straight to Fort Dawnguard and start it right at the beginning. I wouldn't recommend that, though. Dragonborn starts after you've been to Ustengrav for the Greybeards.

Both are geared to fairly high level characters, so keep that in mind. Especially Dragonborn, some of the creatures on Solstheim will annihilate low level characters. Dawnguard isn't as bad, but from about level 7 and up, vampires will attack cities and towns at night, potentially killing quest givers until its main quest is complete. So long as you avoid civilization at night, though, you should be fine.

Also, from a story standpoint, I feel like Dragonborn works best if you wait until after you defeat Alduin in the base game's main quest. The antagonist of Dragonborn has unique dialogue based on how many dragons you've slain beforehand. The way it happens, it just seems like it would be best to do it after the main quest.

You can start the Dawnguard DLC almost immediately, but considering you will be knocking heads with some VERY powerful vamps, I wouldn't jump into it until I was at least level 10-20. Dragonborn DLC can't start officially until after you've gone to see the Greybeards. You can actually go to Solstheim before that,

If you have both DLC's installed you will start to get vampire attacks at around level 10 so it's best to start the Dawnguard quest around then. With regard to Dragonborn it's probably best to complete the main quest in the base game and then go to Solsthiem at around level 20, anything lower than that and your character may struggle with the enemies on the island.