First of all, the term "bastard sword" can be a bit problematic. There wasn't a really clear definition of what that really meant. There were no centralised consensus saying "if a sword is x meters and weighs y kilograms, it is a bastard sword, otherwise it is not." A good example is the Italian sword fighting treatises. Many authors simply referred to their weapon as "spada", which just means "sword" in Italian. The sword shown in Fiore dei Liberi's treatise is clearly a longsword, while Ridolfo Capoferro taught the use of the rapier. Both called their weapons "spada".

As for what is commonly called the "hand-and-a-half sword" or "bastard sword" in modern fantasy media, it would have been used in a very similar manner as the longsword. Keep in mind that the longsword is primarily a two-handed weapon. Though it can be used in one hand, It is not usually used that way, despite what many video games tell you. It is an extremely versatile weapon, and there are many late medieval treatises describing its use.

Obviously, the two edges of the sword can be used to cut, and the point can deliver thrusts. The pommel can be used as a club, and the hand guards can be used to hook the opponent. Medieval swordplay often included grappling, which is used when the enemy gets really close. When fighting an armoured knight, the sword is used in a stance called the "half sword", where you hold the handle with your right hand, but grab your on blade with your left hand, so the sword can be used like a short spear. This is because a sword cannot cut plate armour, despite what Hollywood might show you.

Here is a good video that shows unarmoured combat, and here is one on armoured fighting.

If you are interested to learn more, head over to r/wma. Hope this helps!