Start the game as Álmos Árpad of the Mogyër in 867, immediately check to make sure you have the "Organizer" commander trait. This gives you a +25% movement speed bonus. Other potential commanders in your court might have it but you can't be sure that any of them have gone down the Strategist Martial lifestyle and picked "Organized March" for an additional 15% movement speed so if you're trying to play optimally you should restart until Álmos or someone with the "Strategist" trait has it. If Álmos does have the organizer trait you can reset your perks to make sure he gets organized march (ideal balance of perks is probably full Strategist + stalwart leader, and ideal lifestyle is strategist).

Recruit your most competent guest to max out commanders and build men-at-arms regiments consisting of three horse archers and one light infantry. Then take the "Prepare to Cross the Carpathians" decision.

Declare war on King Boris of Bulgaria using the "Migrate to Pannonia" CB that you just unlocked. Event troops will pop up in your capital of Yary, but you can just delete them and reraise all of your men in the Tribe of Odessa (to easily get them to appear in a single county you can hold down the control key while you raise them).

Making sure you're in command, order your troops to march towards the Bulgar capital (as long as you cross the Danube at the closest crossing point to the Black Sea it's okay) and unpause.

You should see the Boris-commanded Bulgar army in the county of Silistra where you can engage them. If they try to run away you can chase them and you'll catch them fairly quickly. If they're in Tutrakan or Shumen and heading towards Silistra they should be movement-locked; otherwise try to chase them or give up the run. Sometimes Bulgaria will ally Slavonia or another duchy in the west and their army will be nowhere to be seen.

The base chance to capture the enemy commander is 10%, but it goes up to 50% if it's a stackwipe. If you successfully capture Boris you should pause as soon as you do (I'd recommend running the game on speed 1 during the battle's pursuit phase), enforce demands, and convert to Orthodoxy via the religions tab (Religions button > Other Faiths > "Only Uralic Faiths" > Orthodoxy > Convert to Faith).