I completely agree with Lane. Doing it all in one day is impossible. Plan a day for Ljubljana, one for Bled and Bohinj and another day for the drive itself, you will not disappointed.
The classic route is as follows: Follow the A2 motorway to Jesenice, get off the motorway at exit no. 1 (Jesenice zahod), from where you should take road 201 to Kranjska Gora. Once you reach Kranjska Gora, turn left towards Bovec and Vrsic Pass to road 206. After you've crossed Vrsic Pass you will be in the Soca Valley so just follow the river, so to speak. Bovec and then Kobarid will be signposted through the whole upper valley. Unfortunately should you be in a hurry and want to get to the Soca Valley ASAP, the "direct" route from Lake Bled is actually even less direct than the Vrsic Pass, you would have to get to Kranjska Gora and continue into Italy and then drive over the Predel Pass to Bovec. Another winding high-altitude road, although slightly lower and wider than the Vrsic Pass. A note on navigating, road numbers are only shown on motorway direction signs, once you get on the regular roads, only destinations are shown. You can still verify you are on the right road by checking the kilometer markers, though. Those show the road number.
As for accommodation reccommendations, now that the two major hotels (Alp and Kanin) in Bovec are in the process of bankrupcy, there are not too many options. Hotel Dobra Vila and Hotel Mangart in Bovec and Hotel Hvala in Kobarid are the only worthwhile options. That said, the Soca Valley is full of private holiday rentals, that seems to be the most popular option. Or, alternatively, if you consider yourself foodies, Hisa Franko on the outskirts of Kobarid is one of the best restaurants in Slovenia and offers accommodation options, too. Although if you decide on that one, book early. The place and its chef, Ana Ros, will appear on Netflix's Chef's Table this season so they're probably going to be in very high demand in the next few years.
Out of curiosity, what are your plans between the Soca Valley and Croatia? I am very familiar with that area due to family ties and would be more than happy to give some pointers.