It is hard in the early months, but what l used to do is follow the gina Ford routine with some tweaks that suited my DD, ( l NEVER left her to cry)
So up at 7.30 feed, burp nappy, chatting/singing, playgym, not being held all the time but being nearby, 9.30 feed, nappy, nap time in cot in dark room, to get her used to self settling.
l would lie her down with my arms encircling her and shushing constantly until she dropped off, (This did entail being on my knees beside her cot for 30 minutes)
She would sleep for an hour at that age then play/cuddles/playgym etc, often l would put her down wherever l had jobs to do and chat to her whilst l did them.
12.30 feed, nappy, in cot for nap as before, once she dropped off l would try and slip out, if she woke l would sit on the floor by the cot with one arm around her, (and a book in the other!) shushing until she dropped off
Sometimes l spent the whole time there but as time went on, l could take my arm back, but stay in the room, then move to doorway, sit and shush. By 4 months (16 weeks) she would only take a few minutes to go off to sleep, and l could leave. This only really works if it is a consistant routine. I preferred to have a happy good sleeper to being out and about.
Lunchtime naps lasted until 2pm ish , so feed, nappy, play, bath or shower with me, feed and short held nap for 15 minutes at 5pm.
Bedtime at 7.30pm. with sleepy feed in dark room at 10.30pm and again at 4.30pm. But always up to start the day at 7.30 or it didn't really work at all.
By 5 months she could be dropped gently into her crib in a dark room for all her naps and left immediately without ever crying, and l had the hour 9.30 -10.30 and lunchtimes from 12.45 till 2.30 ish to myself.
Plus evenings from 7.30pm..
But not all babies are the same it took my son a bit longer to settle into a good routine say 6-7 months.
It is wrong to leave small babies to cry, but fine to very slowly let them self settle in tiny increments. It is also finer to not hold them all the time, as long as they can hear you nearby they are often fine for a while. I used to leave my son in his basket on a wide kitchen island looking up at the sky and trees, and he would happily gurgle away whilst l cooked or cleaned for up to an hour...
Some babies just won't be happy though so then a sling is good, as pp said, although l have a back injury so that was not an option for me.