How to make an interface?

 I just hopped into this of doing mods and all so i was trying to get a "woodcutter" (i recently discover there is a mod about it but i still want to give it a shot). The problem is i dont know how to make the interface of it. I made the basic GUI (two slots one for in (theres an image of it)) but i dont know how to make the part of the middle to select what block do you want (a stair or a button or something like that) anyone knows how can i do it? or how can i make progress on it, anticipated thanks :D