Hi! I started my account on Splinterlands in August 2021, and here's what I've learned after a month: you CAN rent for profit. In my experience, I've achieved Diamond 3, but the rent was expensive (more than I could make daily) but I got lucky and made back (most of) my rentals through 2000 DEC drops in my daily quest rewards.
However, as a beginner, I personally think you have to progress slowly and steadily in this game. Since this is a beginner's guide, it will cover collection power, budget rental strategies, and ERC management.
The table looks intimidating at first, but it's necessary when planning to rent out cards for the first time. Special shout out to 
Power = (Regular or Gold Foil) * Rarity * Edition * BCX
example: "I want to reach Silver 3, which needs 15000 power."
possible rental options:
A. Gold Foil Epic Beta/Rewards(1st Gen), BCX = 1
B. Regular Foil Legendary Untamed/Dice/Rewards(2nd Gen), BCX = 1
C. Regular Foil Rare Untamed/Dice/Rewards(2nd Gen), BCX = 375
D. Regular Foil Common Rewards(3rd Gen), BCX = 3000
Now, you may be wondering how does BCX work? 1 BCX = 1 card.
Each rarity will require a different number of cards to level them up, and each level represents the total number of cards burned to get that level. So that 3000 BCX for choice D doesn't have to be just one card!
example: Let's take the Goblin Shaman table from https://peakmonsters.com/rentals
ALPHA Goblin Shaman level 1 (BCX = 1) will give 30 power
ALPHA Goblin Shaman level 5 (BCX = 19) will give 30 * 19 = 570 power
BETA Goblin Shaman level 1 (BCX = 1) will give 15 power
BETA Goblin Shaman level 5 (BCX = 25) will give 15 * 25 = 375 power
So keep this in mind as you rent cards so that you'll get the target power level for your target league.
Depending on the size of the season reward pool at the time you play, At Bronze 3, average rewards are close to 0.3-0.8 DEC. At Silver 3, average rewards are close to 5-10 DEC.
If you intend to play to make at least 15 wins per 24 hour period, this will equate to:
BRONZE: 4.5 DEC/15 wins
SILVER: 75 DEC/15 wins
(not including the rewards for your daily chest)
These values now give us an approximate budget PER DAY when renting cards to win. Now we should also take note of the summoner caps per league.
Bronze:
Silver:
I made a quick reference table on the playable monster levels, depending on the level of the summoners:
So putting all this info together, if you want to play competitively at the Bronze league, your RARE summoners have to be AT HIGHEST level 2, so that you can play the maximum level that the league allows. For silver, RARE summoners have to be AT HIGHEST level 4, so that you can play the maximum level that the league allows.
To clarify, if you are at Bronze league, renting a MAX LEVEL RARE summoner will NOT allow you to play monsters any higher than C3, R2, E2, L1. This also means that no matter how high your monster levels are, they will be capped at C3, R2, E2, L1. In-game, this is denoted with an exclamation point on the monster card. HOWEVER you can still rent higher power monsters and play them at the lower levels, just in case you would want to advance to the next league (especially if you sniped them at cheap rental rates).
Some players stay at the bronze/silver leagues even with millions of power in their collections because of the season rewards. So even at the lower leagues, the game gets SUPER competitive.
Bronze league gives out 30000 DEC for Top 1 and 2000 DEC for Top 20
Silver league gives out 40000 DEC for Top 1 and 4000 DEC for Top 25
(Gold, Diamond and Champion leagues have their own, much higher, season rewards)
After much discussion with many different players, it seems there are three ways to rent "efficiently" based on your budget.
Now with the new updated way they calculate season rewards, option 3 might seem a little risky, since you may find the rental prices SPIKE HARD as the season ends. Option 1 is probably the most secure way to ensure your deck until the end of the season, but remember that the rentals can be canceled by the owner at any time so you might want to check your rental contracts (easiest to do on peakmonsters) at least once a day to see how much more time you have on rented cards.
ECR (Energy Capture Rate) - basically the game's way to prevent unlimited grinding. The higher your ECR, the higher your DEC rewards (relatively). The ECR recharges at a rate of approximately 1% per hour, so from 100%, you can essentially play until your ECR drops to 76% and then wait until the next day to play again. This way, you can keep your DEC earnings relatively high.
Another strategy that most players use is to play from 100% until around 52% ECR. This strategy is favored by most renters who choose option 3 because of how the Daily Quests stack. Each Daily Quest will start its countdown after ONE RANKED BATTLE is played after the (new) Quest loads. The Daily Quest will renew after 23 hours, so essentially you have 2 Daily Quests stacked consecutively. It won't stack any more than that because the 3rd DQ counter won't start until you start one battle when the 2nd (stacked) one loads.
So my strategy this last season was to grind my way to the highest rating I could until my ECR hit ~52%. So when tomorrow's quest loads, I will play one battle (win or lose) and then start to look for cards to rent. It will take some time to rent out the deck that you want (with careful planning of course), but you should already have a list ready. This way, when my ECR charges back up to 100%, I'm ready to rent a day's worth of DEC, play 2 daily quests, and then rinse and repeat.
Of course, this strategy takes a lot more effort than just renting for 15 days and playing until 75% ECR, but this saves about half of the DEC you'd spend on the downtimes while waiting for your ECR to charge.
I will probably be posting Bronze/Silver level strategies for each splinter soon, like which cards are used for lower levels and placement strategies during battles.
I wish you all the best of luck and may it rain Gold Foil luck in your daily chest rewards.
@cutedawako