There is very likely not a reliable scientific guide to making and keeping friends, because it depends very much on the circumstances and the people involved.

Some people have many shallow friendships, others just a few deep ones.

Some people extend the hand of friendship early, others take years of persistence to become friends with.

Some people value time spent together on shared experiences, others value talking, others value gifts or help in times of crisis and others value being told how good a friend they are (once the friendship is clearly there).

Many people are friends with individuals they have known since high school and it is generally harder to make new friends after that point. It's also pretty easy to lose friends as you age due to differences of opinions or beliefs.

Perhaps the biggest skills in making and retaining friends is finding common activities and experiences you can share with them (outings, games, sport, dining, interests, topics of discussion, etc.) and finding the right time to invite them to engage with you in such things.

Very often there are logistical hurdles to overcome and that is all: do they have a car, is there a convenient bus, is the venue you are meeting nearby, are you available when they are, do they have a babysitter, is the suggested date suitable for them, have they currently got some other crisis or deadline to deal with, etc.

I find that if I ask to spend time with people too often, they may think I am "clingy" or "needy" and push me away. If I ask too little, the friendship never really gets established. It's a balance that is different for everyone.