Usually when we make a table, the legs are 2 x 2 and the cross members are rectangular, i.e. 1.5 x 2.5 or similar.

To center the cross member in the legs, I lay the parts flat on the table, then place several fender washers on the cross member as shown until the washers are flush with the top of the leg:
Attachment 1280

I then take half of them and place them under each end of the cross member to center it in the leg:
Attachment 1281