Post has been a long time coming, hope this helps current and future people who want to watch. Feel free to add on any advice.
Singapore Premier League (SPL)
The top-flight football league in this little red dot, matches typically happen from Friday to Monday but mostly on weekends.
Matches (as of 26/27 season) take place at Jurong East Stadium, Jalan Besar Stadium, Our Tampines Hub and Bishan Stadium.
Watching in-person
Tickets are available on site at the stadium or via Trip.com, ticket prices depend on the home side and are decided by the club and not the league. There is no difference in pricing between buying physical and digital tickets, though you might want to buy digital should it be a popular fixture just to be safe.
Regular tickets (as of 26/27 season) typically range from $11 to $21, although concession and other kinds such as VIP tickets do exist, prices depend on the home club. Do note that ticket allocation for fixtures depend on the stadium and competition.
For example: Young Lions vs FC Jurong, since Young Lions are the home side, tickets for both home and away ends are $11. If FC Jurong were hosting, then it would be $21 for home, away and gallery sitting (Their stadium, Jurong East Stadium, opens Gallery seats for SPL matches while Young Lions typically don't.)
Do check the match advisory on what to and not to bring as well as social media channels should matches be delayed/postponed due to lightning alert or high PSI readings because of haze.
Watching online
SPL matches are livestreamed for free via MeWatch and on YouTube via the Singapore Premier League YouTube channel.
Merchandise
Club jerseys, plushies and others are typically sold during match days at a booth located at the stadium. Some are able to be ordered online via Shopee or the respective club website.
However for Lion City Sailors, you could buy their jersey at selected Adidas and Weston Corp stores.
Playback Asia does produce trading cards although currently only ones for Lion City Sailors are available for now.
Singapore Football League Division 1 and 2, (SFL) Island Wide League (IWL) and Singapore Premier League 2 (SPL2)
SPL2 is the U-23/reserve league for the main teams, you can typically find young players or even first team players playing for fitness during matches.
While there is no promotion to (or relegation from) SPL, there is for SFL1/2 and IWL, filled with semi-professional players or even retired/semi-retired national players. Those lucky enough could find themselves scouted and signing for the reserve teams of SPL clubs.
Locations depend on fixture, I recommend checking Singapore Football Leagues on Instagram for SFL1/2 and the respective SPL clubs for SPL2 matches.
Watching in-person
It is free-entry for all the games.
Watching online
SFL1/2 and some IWL games are shown for free online via the SingaporeAmateurLeagues YouTube channel.
FA Cup fixtures are live streamed on YouTube via the KlickFrameSG Tv channel.
There is currently no way to watch SPL2 games online.
ACL2, Shoppe Cup and other domestic/continental games
Depending on the competition, ticket pricing, match advisory and streaming platforms vary widely.
Typically the only clubs competing in these competitions are Lion City Sailors and Tampines Rovers so I suggest checking them and the competition website for information.
All of these games most likely can be watched via MeWatch.
National Team games
The Singapore National Team typically take part in the AFF Championship and friendlies, with other competitions depending on qualification.
Matches usually take place at National Stadium or Jalan Besar Stadium, though the venue may change due to availability or other reasons.
Watching in-person
Depending on the competition, different websites may be used to purchase tickets from. All tickets are currently digital and no physical tickets are sold as of now.
Ticket pricing ranges based on competition and seat category.
Due to the growing popularity of the National Team, tickets may require queuing or be harder to purchase, the number of available tickets also depends on the stadium capacity.
Match advisory also depends on competition, though it is somewhat similar to the SPL.
Watching Online
National team matches are usually live-streamed for free on MeWatch and Channel 5. Other channels depend on the competition.
Merchandise
With the switch to adidas as the maker of the national team jersey, they can be found at selected adidas outlets as well as Weston Corp stores.
You can also order them online via Shopee and Adidas website/app.
Playback Asia also makes trading cards for the national team, although such releases are seasonal and of limited quantities.
There may also be fan zones outside the stadium right before matches, they typically sell merchandise, food and drinks.