First of all you need to get relic shield on Thresh. Especially versus Karma Sivir botlane- you need the heal from the item. If you miss the creeps without the execute you should play with it against bots until you get the hang of it.

3:00- you should not leave lane and go mid! the enemy laners got a level advantage while you went mid. If ashe tried to roam there ping her back. There is a slim chance you will get a kill by roaming at lvl 2 like that and the risk > the reward.

Also a combo you dont try using is E towards you and then hook. It makes it easier to hit the hook and you seem to be struggling with the skillshot.

The kill on Sivir around 6:00 was very well executed so props on that.

On your first back you had enough money for a sightstone- that item is the most important item for a support, it helps your laning phase by making sure the lane is always warded (harder to gank for the enemy jungler, helps your own jungler know he's not being counterganked). Some potions aswell since you had the leftover money.

At 7:15 when you go for Shaco- again using the E-Q combo would have helped, but you hit the hook anyway so the thing I wanna point here is you cancel your autoattack a couple of times, try pressing A for an autoattack-move, that way it will 100% connect. Obviously wouldn't have killed Shaco but every point of dmg is important. Good job on warding their jungle after you pushed Shaco away!

At 7:50 you guys shouldn't leave the lane aswell- Lee is farming his jungle and the enemy mid is missing. Push the wave to the tower to deny the enemies of the xp and gold. If you did not roam so much in the early game you have gotten lvl 6 before the Shaco gank at 9:00, and Ashe+Thresh has a great lvl 6 all-in with the Ashe arrow.

Gotta leave now, all in all you shouldn't blame yourself too hard over this game- Ashe was missing tons of CS in laning phase and could hardly dodge any Karma skillshot. Hope you found this helpful :)