Welcome.
Firstly, lucky you if you've got a plentiful supply of cheap or free wood! Secondly, be aware that branch wood is going to be full of stresses and most of it is going to be unsuitable for any situation where one piece of wood joins to another. Thirdly, if this wood is newly cut, then it is "green", and will shrink and crack as it dries. A lot. Often rendering it unusable.
If you get past all those issues and have anything usable left, then there are all sorts of ways of flattening wood, from an adze or a hatchet at one end of the scale, to a planer thicknesser at the other. In between lies hand planes...but they're only any use if you've got a solid worksurface and a means of restraining the wood from moving as you work. Unfortunately, "easiest" and "cheapest" are in a strong inverse relationship with this subject. The cheapest way is going to involve more sweat than the easiest way, but be a matter of a few pounds of investment rather than possibly thousands.
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