The best way to assemble most cabinets is to lay one of the two side panels down flat on a level table, then snap all the horizontal parts into the side panel (they will be horizontal once you stand the cabinet up into it's upright and normal position).
Next, lay the second side panel on top of the horizontal parts, and align all the fittings, then you can gently tap them with your fist or the dead blow mallet (don't try to force them).
Then use the dead blow mallet to lightly tap all your parts into alignment (the Lamello clips allow approximately 3mm (1/8") of lateral adjustment).
For a step by step look at the assembly process with images, take a look at our Blog post on Full Access Assembly.