Saw off the bottom of the trunk 1-2 inches and use a drill to bore a narrow hole up through the middle of the trunk. Fill the hole with cotton, tightly packed. Put the base in warm water as soon as possible.
Warm water will melt the sap and allow the tree to drink more easily. It’s been recommended to add one of these to the water as a preservative: a few tablespoons of glycerin, or a few tablespoons of sugar, a few crushed aspirins, or use one of the pre made”tree preserve” mixtures.