Unless you specified a base removal step, the only boundary condition that must be applied is a fixed support boundary condition on the build plate. If you want to remove the base, you need to apply fixed boundary conditions on three nodes of the part to prevent rigid body motion when the base is removed.
Procedural Steps
To fix the bottom surface of the base plate, right-click the Static Structural object and then select Insert > Fixed Support. Select the bottom surface of the base plate (geometry selection) and hit Apply in the Details view. If you have a more detailed geometry for the base, such as one with bolt holes that you want to explicitly simulate, apply the boundary conditions to affix the plate accordingly.
If you specified a base removal step, you must constrain the part at three nodes to prevent rigid body motion. Be aware that you are constraining the nodes to their displaced position at the end of the build, rather than constraining them to 0. (For example, if the node displaced 0.1 mm during the simulation, the constraint will keep the node at 0.1 mm.) Use a Commands object to do this.
You may want to hide the base body during this process. Right-click the base body and select Hide Body.
Zoom in to the area of the part where you will apply constraints. Switch to Node mode of selection.
Create nodal named selections: Right-click Model in the project tree, and select Insert > Named Selection. Select a node on the bottom of the part, perhaps toward the middle of the part. You should choose nodes that are unimportant to your results. Perhaps spread them out over the bottom of the part but the nodes must not be co-linear. Your node selection will affect displacement results but not stresses or strains. Click Apply in the Details view. Right-click the newly created Selection in the project tree (under Named Selections) and select Rename. Name it to DED_UX_Remove (for constrained node 1). Do the same step two more times and rename them to DED_UY_Remove and DED_UZ_Remove.
Remember to make the base plate visible again if you hid the body. Right-click the base body and select Show Body.