You must first build a construction yard in whatever castle you wish to use to build items.  The construction yard is available at level 45 and will allow you to make use of the materials like sand and bricks you have received from material sacks. You get materials from participating in events and from nobility contests. Material sacks can also be purchased with rubies.

So what you do is you click on the building or the construction tab and you will see the blue box that says build items. click that and there will be a menu there. click the bottom tab that says "craft build items'' then you use the top right drop down menu to select what type of building you want to make the build items for. So you would select whatever building you want.  Then you go to the bottom right and hit the green "CRAFT" button. that will make you a level one build item which will boost production by +20. (other buildings have other bonuses like additional storage space etc. ) but that's not good enough. so if you look in the area to the bottom left where it says level 1. click the right arrow and it will show level 2 which is + 40. then click the green CRAFT button again. repeat for level three build item which is the max without research which is +60 res per hour in most cases. You can also use build items for the Marketplace which will increase your number of market barrows which is very useful. Anyway. click the top tab Assign Build Items. Then use the second drop down menu to select whatever building type for which you just made the build items in the craft screen. You will see all the buildings of that type on the bottom left. Click one and it will appear on the right while below it there will be the build items you just made. From here drag and drop the build item hexagon into the hexagon slot on the left below the picture of the building and it will ask if you want to assign it. Click the green yes and it will apply the bonus. This is not permanent and you can remove them later to upgrade them at the cost of coins.