The first few times are not “easy” but, with patience, consideration, and care the first times don’t have to be painful or difficult. You say you know about lube which is good. Use lots of lube, and then use more. Other tips:
Take plenty, plenty, plenty of time. Don’t get impatient or rushed.
Let the receiving partner set the pace. Follow their lead.
Use your fingers and tongue to stimulate the receiving partner before attempting penetration. This will give them time to relax and learn how pleasurable anal play can be.
Let the receiving partner be on top at first. This allows then to control the depth and speed of penetration.
When the penis starts to penetrate the anus, have the receiving partner push down like they are trying to poop. This helps open and relax the sphincter muscles.
Play with, or have the receiving partner play with, their clit (or penis) during penetration. This will greatly increase their pleasure and likely lead to a very intense orgasm for them.