Susan Witmore wrote a step-by-step guide on how to build your own igloo. You should read it, just in case...
Construction begins with the cutting of snow blocks. In most areas snow falls without compacting enough to allow blocks to be cut. Tramp an area the size of your intended snow block quarry for at least 15 to 30 minutes, then let it rest a half hour. Compaction causes the small ice crystals of snow to melt. These quickly refreeze, forming a more solid building material. The size of the blocks you cut will depend upon two things:
1. How heavy a block are you able to comfortably handle? Your strength and the moisture content of the compacted snow will provide some practical limits to the size of the block.
2. How strong is the compacted snow? In areas where layers of snow have thawed and refrozen, there may be ice layers in the snow. These layers make the snow blocks fragile. If such blocks must be used, they will have to be thicker than those cut from blocks without ice layers. Well compacted, low moisture snow can be cut into large thin blocks.