Here's a big resource for dealing with MtF or transfeminine dysphoria and how to pass (if that's what you want). Not all trans people want to pass, but here are some tips and guidelines for those who do.
Tucking is the act of creating the illusion of having a vulva by tucking the penis beneath you and pushing the testicles into the inguinal canals.
How to Tuck
It's suggested that you have a warm shower before tucking. This caused the skin to become softer and thus more elastic. You can either start with the testicles or with the penis - whatever works best for you.
Testicles
Gently push the testicles upwards and in. Do not rush this step
Penis
Guide the penis as far underneath you as is comfortable, then guide it gently upwards until it stays put.
Everything can be held in place using with regular underwear or a gaff
Important to remember
DO NOT USE TAPE. It forces the testicles into an unhealthy position and long term exposure to this may cause harm. If you absolutely must use tape, make sure it is medical tape. This is more suitable for sensitive skin. Still consider potential harm.
DO NOT EXERCISE WHILE TUCKING. This may lead to chaffing or injury.
Tucking may be mildly uncomfortable, but it shouldn’t be painful. Don’t force your genitals to move. If you’re having difficulties or experiencing a lot of discomfort, stop. Take a break, and return later.
Take a break when you’re alone or wear loose clothing when it’s not as important to be tucked. To avoid chafing or prevent a skin infection, always check for any open or irritated skin before and after tucking.
Tucking may affect your fertility. For trans folks who may want to have biological children someday, it’s recommended you tuck sparingly or (if you’re able to afford it) consider banking sperm.
Tips
Practice tucking a few times when relaxed and in a comfortable space at home before going out. This can help you to avoid any panic or stress in public if it’s your first time tucking.
If you can't or shouldn't tuck wearing a jumper around your waist can be almost as effective
For more information about tucking visit Healthline
For many transfeminine folk having underclothes that affirm gender identity is very important. "A gaff is, at its most basic, a pair of compression underwear. A gaff is made to be worn either with or without a “tuck” and works to help trans femme folks (or anyone else with a penis) achieve a flatter, more gender-affirmed look". - Point of Pride. This can help with passing in public which can be really important in terms of safety.
Gaffs come in many different sizes but choosing the right size is as simple as choosing a regular pair of underwear. Meaure around your waist (just a bit above your hips) then check the seller's sizing guide. DO NOT go down a size, tighter isn't better and may damage you.
Alternatives to official gaffs
A swimsuit bottom makes a good alternative for a gaff.
You can also use panties made of tightly woven Spandex or a microfiber blend.
Stay away from silk. Natural fibers can be “slippery” and can cause things to move once you have them in place, which can be uncomfortable.
Cotton is a great material for breathability and price, but has a tendency to stretch out and lose its shape over time.
Gaff tips from Point of Pride
No matter what tucking garment you pick, if you do tuck, make sure the fabric is tight and thick enough to hold everything together discretely, yet thin enough to feel comfortable and breathable beneath your clothing.
Things may be uncomfortable, but you should never feel pain. If you ever feel pain, stop and take a break!
Remember, you can wear and do whatever you want and still be valid as a trans woman, but here are a few tips that can help with dysphoria and help you pass in public:
Clear nail polish (or coloured if you'll be safe!)
Shave and take care of your feet
Look in the mirror and pick out your favourite/ most fem features of yourself
Lip gloss or lip balm
Wrap the towel over your chest when you get out of the shower
Sweater paws! - putting your sleeves over your hands
Skincare - try using moisturiser to make skin softer
Using hair clips similar to your hair colour
Try to walk with your hips more than your shoulders
Try to talk softer and with your head voice (speaking where it resonates more in your head than in your chest). Voice Pitch Analyser is a great app to find out how your voice is perceived as for as gender goes.
Try talking with your hands more
Try not slouch. Cross your legs or if you don't feel comfortable doing that, try to close your knees a little more. Leaning into a conversation more tends to come off as more feminine
If you're going to wear shorts, try not to wear ones that come lower than two thirds of your thigh
Tighter clothing
Any feminine feeling clothing works